N-CSRS

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM N-CSR

CERTIFIED SHAREHOLDER REPORT OF

REGISTERED MANAGEMENT INVESTMENT COMPANIES

Investment Company Act file number 811-21780

MFS SERIES TRUST XII

(Exact name of registrant as specified in charter)

111 Huntington Avenue, Boston, Massachusetts 02199

(Address of principal executive offices) (Zip code)

Christopher R. Bohane

Massachusetts Financial Services Company

111 Huntington Avenue

Boston, Massachusetts 02199

(Name and address of agents for service)

Registrant’s telephone number, including area code: (617) 954-5000

Date of fiscal year end: October 31*

Date of reporting period: April 30, 2018

* This Form N-CSR pertains to the following series of the Registrant: MFS Equity Opportunities Fund. The remaining series of the Registrant each has a fiscal year end of April 30.

ITEM 1. REPORTS TO STOCKHOLDERS.

Semiannual Report

April 30, 2018

MFS® Equity Opportunities Fund

MSR-SEM

MFS® Equity Opportunities Fund

CONTENTS

Letter from the Executive Chairman	1
Portfolio composition	2
Expense table	3
Portfolio of investments	5
Statement of assets and liabilities	8
Statement of operations	10
Statements of changes in net assets	11
Financial highlights	12
Notes to financial statements	22
Proxy voting policies and information	32
Quarterly portfolio disclosure	32
Further information	32
Information about fund contracts and legal claims	33
Provision of financial reports and summary prospectuses	33
Contact information    back cover

The report is prepared for the general information of shareholders.

It is authorized for distribution to prospective investors only when preceded or accompanied by a current prospectus.

NOT FDIC INSURED • MAY LOSE VALUE • NO BANK GUARANTEE

LETTER FROM THE EXECUTIVE CHAIRMAN

Dear Shareholders:

Rising bond yields have led to a measurable uptick in market volatility — a departure from the low-volatility environment that prevailed for much of 2017. In recent months,

against this backdrop, global markets have given back some of the strong gains recorded during 2017 and early 2018. Global economic growth remains robust, notwithstanding signs of a modest slowdown over the past few months, particularly in Europe.

Although the U.S. Federal Reserve continues to gradually raise interest rates and shrink its balance sheet, monetary policy remains accommodative around the world, with many central banks taking only tentative steps toward tighter policies. Newly enacted U.S. tax reforms have been welcomed by equity markets, while emerging market economies have benefited from a weaker U.S. dollar.

Around the world, inflation remains largely subdued, but tight labor markets and solid global demand have investors on the lookout for its potential resurgence. Increased U.S. protectionism is also a growing concern, as investors fear that trade friction could disrupt the synchronized rise in global growth.

As a global investment manager, MFS® strives to create long-term value and protect capital for clients through an active approach and an investment platform built on nearly a century of expertise. To make that long-term value meaningful for clients, we work to align with you on our beliefs, your needs and the time it takes to deliver on your desired outcomes.

Respectfully,

Robert J. Manning

Executive Chairman

MFS Investment Management

June 18, 2018

The opinions expressed in this letter are subject to change and may not be relied upon for investment advice. No forecasts can be guaranteed.

1

PORTFOLIO COMPOSITION

Portfolio structure

Top ten holdings
Marathon Petroleum Corp.	3.2%
Take-Two Interactive Software, Inc.	3.0%
Phillips 66	3.0%
Urban Outfitters, Inc.	2.9%
Owens Corning	2.8%
DXC Technology Co.	2.8%
East West Bancorp, Inc.	2.8%
Archer Daniels Midland Co.	2.6%
Unum Group	2.6%
Store Capital Corp., REIT	2.6%

Equity sectors
Financial Services	24.9%
Industrial Goods & Services	13.0%
Health Care	11.0%
Utilities & Communications	8.8%
Technology	8.1%
Consumer Staples	7.4%
Energy	6.1%
Special Products & Services	5.1%
Leisure	4.8%
Retailing	2.9%
Autos & Housing	2.8%
Basic Materials	2.5%
Transportation	2.3%

Cash & Cash Equivalents includes any cash, investments in money market funds, short-term securities, and other assets less liabilities. Please see the Statement of Assets and Liabilities for additional information related to the fund’s cash position and other assets and liabilities.

Percentages are based on net assets as of April 30, 2018.

The portfolio is actively managed and current holdings may be different.

2

EXPENSE TABLE

Fund expenses borne by the shareholders during the period, November 1, 2017 through April 30, 2018

As a shareholder of the fund, you incur two types of costs: (1) transaction costs, including sales charges (loads) on certain purchase or redemption payments, and (2) ongoing costs, including management fees; distribution and service (12b-1) fees; and other fund expenses. This example is intended to help you understand your ongoing costs (in dollars) of investing in the fund and to compare these costs with the ongoing costs of investing in other mutual funds.

The example is based on an investment of $1,000 invested at the beginning of the period and held for the entire period November 1, 2017 through April 30, 2018.

Actual Expenses

The first line for each share class in the following table provides information about actual account values and actual expenses. You may use the information in this line, together with the amount you invested, to estimate the expenses that you paid over the period. Simply divide your account value by $1,000 (for example, an $8,600 account value divided by $1,000 = 8.6), then multiply the result by the number in the first line under the heading entitled “Expenses Paid During Period” to estimate the expenses you paid on your account during this period.

Hypothetical Example for Comparison Purposes

The second line for each share class in the following table provides information about hypothetical account values and hypothetical expenses based on the fund’s actual expense ratio and an assumed rate of return of 5% per year before expenses, which is not the fund’s actual return. The hypothetical account values and expenses may not be used to estimate the actual ending account balance or expenses you paid for the period. You may use this information to compare the ongoing costs of investing in the fund and other funds. To do so, compare this 5% hypothetical example with the 5% hypothetical examples that appear in the shareholder reports of the other funds.

Please note that the expenses shown in the table are meant to highlight your ongoing costs only and do not reflect any transactional costs, such as sales charges (loads). Therefore, the second line for each share class in the table is useful in comparing ongoing costs only, and will not help you determine the relative total costs of owning different funds. In addition, if these transactional costs were included, your costs would have been higher.

3

Expense Table – continued

Share Class		Annualized Expense Ratio		Beginning Account Value 11/01/17		Ending Account Value 4/30/18		Expenses Paid During Period (p) 11/01/17-4/30/18
A	Actual		1.18%		$1,000.00		$992.01		$5.83
	Hypothetical (h)		1.18%		$1,000.00		$1,018.94		$5.91
B	Actual		1.93%		$1,000.00		$988.07		$9.51
	Hypothetical (h)		1.93%		$1,000.00		$1,015.22		$9.64
C	Actual		1.93%		$1,000.00		$988.40		$9.52
	Hypothetical (h)		1.93%		$1,000.00		$1,015.22		$9.64
I	Actual		0.93%		$1,000.00		$993.22		$4.60
	Hypothetical (h)		0.93%		$1,000.00		$1,020.18		$4.66
R1	Actual		1.93%		$1,000.00		$988.35		$9.51
	Hypothetical (h)		1.93%		$1,000.00		$1,015.22		$9.64
R2	Actual		1.43%		$1,000.00		$990.78		$7.06
	Hypothetical (h)		1.43%		$1,000.00		$1,017.70		$7.15
R3	Actual		1.18%		$1,000.00		$991.73		$5.83
	Hypothetical (h)		1.18%		$1,000.00		$1,018.94		$5.91
R4	Actual		0.93%		$1,000.00		$993.69		$4.60
	Hypothetical (h)		0.93%		$1,000.00		$1,020.18		$4.66
R6	Actual		0.84%		$1,000.00		$994.02		$4.15
	Hypothetical (h)		0.84%		$1,000.00		$1,020.63		$4.21

(h) 5% class return per year before expenses.

(p) “Expenses Paid During Period” are equal to each class’s annualized expense ratio, as shown above, multiplied by the average account value over the period, multiplied by 181/365 (to reflect the one-half year period). Expenses paid do not include any applicable sales charges (loads). If these transaction costs had been included, your costs would have been higher.

Notes to Expense Table

Each class with a Rule 12b-1 service fee is subject to a rebate of a portion of such fee. Such rebates are included in the expense ratios above and are outside of the expense limitation arrangement. For Class A shares, this rebate reduced the expense ratio above by 0.01%. See Note 3 in the Notes to Financial Statements for additional information.

4

PORTFOLIO OF INVESTMENTS

4/30/18 (unaudited)

The Portfolio of Investments is a complete list of all securities owned by your fund. It is categorized by broad-based asset classes.

Common Stocks - 99.7%
Issuer	Shares/Par		Value ($)
Aerospace - 7.0%
Boeing Co.		28,153	$	9,390,714
Lockheed Martin Corp.		27,283		8,753,478
ManTech International Corp., “A”		168,597		9,962,397
			$	28,106,589
Airlines - 2.3%
Delta Air Lines, Inc.		179,300	$	9,363,046
Alcoholic Beverages - 2.2%
Molson Coors Brewing Co.		125,183	$	8,918,037
Biotechnology - 2.3%
Biogen, Inc. (a)		33,050	$	9,042,480
Brokerage & Asset Managers - 2.2%
Apollo Global Management LLC, “A”		302,465	$	8,723,091
Business Services - 5.1%
Accenture PLC, “A”		61,472	$	9,294,567
DXC Technology Co.		108,752		11,207,981
			$	20,502,548
Cable TV - 1.9%
Comcast Corp., “A”		237,681	$	7,460,807
Chemicals - 2.5%
CF Industries Holdings, Inc.		255,133	$	9,899,160
Computer Software - Systems - 3.6%
Hewlett Packard Enterprise		419,654	$	7,155,101
Tech Data Corp. (a)		96,317		7,344,171
			$	14,499,272
Construction - 2.8%
Owens Corning		171,586	$	11,237,167
Electrical Equipment - 2.0%
Hubbell, Inc.		77,475	$	8,046,554

5

Portfolio of Investments (unaudited) – continued

Issuer	Shares/Par		Value ($)
Common Stocks - continued
Electronics - 2.2%
Marvell Technology Group Ltd.		450,177	$	9,030,551
Energy - Independent - 6.1%
Marathon Petroleum Corp.		168,694	$	12,636,868
Phillips 66		106,665		11,872,881
			$	24,509,749
Food & Beverages - 5.1%
Archer Daniels Midland Co.		230,118	$	10,442,755
Tyson Foods, Inc., “A”		145,645		10,209,714
			$	20,652,469
Insurance - 5.9%
Athene Holding Ltd. (a)		182,340	$	8,934,660
MetLife, Inc.		94,837		4,520,880
Unum Group		214,794		10,391,734
			$	23,847,274
Internet - 2.3%
Facebook, Inc., “A” (a)		52,886	$	9,096,392
Leisure & Toys - 3.0%
Take-Two Interactive Software, Inc. (a)		119,237	$	11,889,121
Machinery & Tools - 3.9%
Eaton Corp. PLC		121,902	$	9,146,307
Regal Beloit Corp.		93,582		6,663,038
			$	15,809,345
Medical & Health Technology & Services - 2.3%
McKesson Corp.		57,877	$	9,040,966
Natural Gas - Pipeline - 1.7%
EQT Midstream Partners LP		117,909	$	6,633,560
Other Banks & Diversified Financials - 7.1%
Discover Financial Services		124,553	$	8,874,401
East West Bancorp, Inc.		167,627		11,167,311
Synchrony Financial		260,321		8,634,848
			$	28,676,560
Pharmaceuticals - 6.5%
Bristol-Myers Squibb Co.		156,911	$	8,179,770
Eli Lilly & Co.		95,856		7,771,046

6

Portfolio of Investments (unaudited) – continued

Issuer	Shares/Par		Value ($)
Common Stocks - continued
Pharmaceuticals - continued
Pfizer, Inc.		278,913	$	10,211,005
			$	26,161,821
Real Estate - 9.6%
Brixmor Property Group Inc., REIT		605,832	$	9,020,838
CubeSmart, REIT		327,313		9,636,095
Extra Space Storage, Inc., REIT		106,078		9,503,528
Store Capital Corp., REIT		407,364		10,277,794
			$	38,438,255
Specialty Stores - 2.9%
Urban Outfitters, Inc. (a)		285,386	$	11,492,494
Utilities - Electric Power - 7.2%
Avangrid, Inc.		189,328	$	9,979,479
Exelon Corp.		234,338		9,298,532
PPL Corp.		328,867		9,570,029
			$	28,848,040
Total Common Stocks (Identified Cost, $374,539,958)			$	399,925,348
Investment Companies (h) - 0.5%
Money Market Funds - 0.5%
MFS Institutional Money Market Portfolio, 1.78% (v) (Identified Cost, $1,788,479)		1,788,658	$	1,788,479
Other Assets, Less Liabilities - (0.2)%				(737,971	)
Net Assets - 100.0%			$	400,975,856

(a) Non-income producing security.

(h) An affiliated issuer may be considered one in which the fund owns 5% or more of the outstanding voting securities, or a company which is under common control. At period end, the aggregate values of the fund’s investments in affiliated issuers and in unaffiliated issuers were $1,788,479 and $399,925,348, respectively.

(v) Affiliated issuer that is available only to investment companies managed by MFS. The rate quoted for the MFS Institutional Money Market Portfolio is the annualized seven-day yield of the fund at period end.

The following abbreviations are used in this report and are defined:

PLC Public Limited Company

REIT Real Estate Investment Trust

See Notes to Financial Statements

7

Financial Statements

STATEMENT OF ASSETS AND LIABILITIES

At 4/30/18 (unaudited)

This statement represents your fund’s balance sheet, which details the assets and liabilities comprising the total value of the fund.

Assets
Investments in unaffiliated issuers, at value (identified cost, $374,539,958)	$399,925,348
Investments in affiliated issuers, at value (identified cost, $1,788,479)	1,788,479
Receivables for
Fund shares sold	192,562
Dividends	230,163
Other assets	1,491
Total assets	$402,138,043
Liabilities
Payables for
Fund shares reacquired	$901,273
Payable to affiliates
Investment adviser	33,741
Shareholder servicing costs	208,223
Distribution and service fees	14,184
Payable for independent Trustees’ compensation	1,004
Accrued expenses and other liabilities	3,762
Total liabilities	$1,162,187
Net assets	$400,975,856
Net assets consist of
Paid-in capital	$338,842,811
Unrealized appreciation (depreciation)	25,385,390
Accumulated net realized gain (loss)	35,546,025
Undistributed net investment income	1,201,630
Net assets	$400,975,856
Shares of beneficial interest outstanding	12,467,328

8

Statement of Assets and Liabilities (unaudited) – continued

	Net assets		Shares outstanding		Net asset value per share (a)
Class A		$165,908,711		5,105,237		$32.50
Class B		10,819,033		351,687		30.76
Class C		74,751,167		2,427,359		30.80
Class I		131,849,056		4,048,188		32.57
Class R1		222,036		7,237		30.68
Class R2		642,951		20,367		31.57
Class R3		1,120,975		34,616		32.38
Class R4		607,171		18,592		32.66
Class R6		15,054,756		454,045		33.16

(a) Maximum offering price per share was equal to the net asset value per share for all share classes, except for Class A, for which the maximum offering price per share was $34.48 [100 / 94.25 x $32.50]. On sales of $50,000 or more, the maximum offering price of Class A shares is reduced. A contingent deferred sales charge may be imposed on redemptions of Class A, Class B, and Class C shares. Redemption price per share was equal to the net asset value per share for Classes I, R1, R2, R3, R4, and R6. Net asset value per share is calculated using actual net assets and shares outstanding rather than amounts that have been rounded for presentation purposes.

See Notes to Financial Statements

9

Financial Statements

STATEMENT OF OPERATIONS

Six months ended 4/30/18 (unaudited)

This statement describes how much your fund earned in investment income and accrued in expenses. It also describes any gains and/or losses generated by fund operations.

Net investment income (loss)
Income
Dividends	$4,410,529
Dividends from affiliated issuers	22,169
Total investment income	$4,432,698
Expenses
Management fee	$1,665,880
Distribution and service fees	731,616
Shareholder servicing costs	238,671
Administrative services fee	38,010
Independent Trustees’ compensation	4,299
Custodian fee	13,806
Shareholder communications	19,039
Audit and tax fees	23,586
Legal fees	2,671
Miscellaneous	79,438
Total expenses	$2,817,016
Reduction of expenses by investment adviser and distributor	(25,027	)
Net expenses	$2,791,989
Net investment income (loss)	$1,640,709
Realized and unrealized gain (loss)
Realized gain (loss) (identified cost basis)
Unaffiliated issuers	$35,574,609
Affiliated issuers	(697	)
Net realized gain (loss)	$35,573,912
Change in unrealized appreciation (depreciation)
Unaffiliated issuers	$(39,550,853	)
Net realized and unrealized gain (loss)	$(3,976,941	)
Change in net assets from operations	$(2,336,232	)

See Notes to Financial Statements

10

Financial Statements

STATEMENTS OF CHANGES IN NET ASSETS

These statements describe the increases and/or decreases in net assets resulting from operations, any distributions, and any shareholder transactions.

	Six months ended 4/30/18			Year ended 10/31/17
Change in net assets	(unaudited)
From operations
Net investment income (loss)		$1,640,709			$1,545,127
Net realized gain (loss)		35,573,912			43,536,292
Net unrealized gain (loss)		(39,550,853	)		38,508,362
Change in net assets from operations		$(2,336,232	)		$83,589,781
Distributions declared to shareholders
From net investment income		$(1,275,074	)		$(2,350,035	)
From net realized gain		(38,232,167	)		(4,101,140	)
Total distributions declared to shareholders		$(39,507,241	)		$(6,451,175	)
Change in net assets from fund share transactions		$(22,540,910	)		$(75,223,638	)
Total change in net assets		$(64,384,383	)		$1,914,968
Net assets
At beginning of period		465,360,239			463,445,271
At end of period (including undistributed net investment income of $1,201,630 and $835,995, respectively)		$400,975,856			$465,360,239

See Notes to Financial Statements

11

Financial Statements

FINANCIAL HIGHLIGHTS

The financial highlights table is intended to help you understand the fund’s financial performance for the semiannual period and the past 5 fiscal years. Certain information reflects financial results for a single fund share. The total returns in the table represent the rate that an investor would have earned (or lost) on an investment in the fund share class (assuming reinvestment of all distributions) held for the entire period.

	Six months ended 4/30/18			Year ended
Class A				10/31/17			10/31/16			10/31/15			10/31/14			10/31/13
	(unaudited)
Net asset value, beginning of period		$35.76			$30.38			$28.78			$28.25			$25.54			$18.99
Income (loss) from investment operations
Net investment income (loss) (d)		$0.14			$0.14	(c)		$0.19			$0.12			$0.09			$0.20
Net realized and unrealized gain (loss)		(0.35	)		5.69			1.50			0.49			2.69			6.58
Total from investment operations		$(0.21	)		$5.83			$1.69			$0.61			$2.78			$6.78
Less distributions declared to shareholders
From net investment income		$(0.08	)		$(0.18	)		$(0.09	)		$(0.08	)		$(0.07	)		$(0.23	)
From net realized gain		(2.97	)		(0.27	)		—			—			—			—
Total distributions declared to shareholders		$(3.05	)		$(0.45	)		$(0.09	)		$(0.08	)		$(0.07	)		$(0.23	)
Net asset value, end of period (x)		$32.50			$35.76			$30.38			$28.78			$28.25			$25.54
Total return (%) (r)(s)(t)(x)		(0.80	)(n)		19.37	(c)		5.88			2.17			10.90			36.11
Ratios (%) (to average net assets) and Supplemental data:
Expenses before expense reductions (f)		1.19	(a)		1.19	(c)		1.20			1.21			1.19			1.28
Expenses after expense reductions (f)		1.18	(a)		1.18	(c)		1.19			1.20			1.19			1.27
Net investment income (loss)		0.81	(a)		0.43	(c)		0.63			0.41			0.32			0.93
Portfolio turnover		57	(n)		121			109			119			106			115
Net assets at end of period (000 omitted)		$165,909			$176,950			$210,858			$218,412			$257,948			$155,571

See Notes to Financial Statements

12

Financial Highlights – continued

	Six months ended 4/30/18			Year ended
Class B				10/31/17			10/31/16			10/31/15			10/31/14			10/31/13
	(unaudited)
Net asset value, beginning of period		$34.05			$28.99			$27.59			$27.21			$24.72			$18.38
Income (loss) from investment operations
Net investment income (loss) (d)		$0.01			$(0.10	)(c)		$(0.03	)		$(0.10	)		$(0.11	)		$0.05
Net realized and unrealized gain (loss)		(0.33	)		5.43			1.43			0.48			2.60			6.37
Total from investment operations		$(0.32	)		$5.33			$1.40			$0.38			$2.49			$6.42
Less distributions declared to shareholders
From net investment income		$—			$—			$—			$—			$—			$(0.08	)
From net realized gain		(2.97	)		(0.27	)		—			—			—			—
Total distributions declared to shareholders		$(2.97	)		$(0.27	)		$—			$—			$—			$(0.08	)
Net asset value, end of period (x)		$30.76			$34.05			$28.99			$27.59			$27.21			$24.72
Total return (%) (r)(s)(t)(x)		(1.19	)(n)		18.50	(c)		5.07			1.40			10.07			35.11
Ratios (%) (to average net assets) and Supplemental data:
Expenses before expense reductions (f)		1.94	(a)		1.94	(c)		1.95			1.96			1.95			2.03
Expenses after expense reductions (f)		1.93	(a)		1.94	(c)		1.95			1.95			1.94			2.03
Net investment income (loss)		0.06	(a)		(0.33	)(c)		(0.11	)		(0.35	)		(0.43	)		0.24
Portfolio turnover		57	(n)		121			109			119			106			115
Net assets at end of period (000 omitted)		$10,819			$12,977			$13,895			$17,022			$18,480			$12,876

See Notes to Financial Statements

13

Financial Highlights – continued

	Six months ended 4/30/18			Year ended
Class C				10/31/17			10/31/16			10/31/15			10/31/14			10/31/13
	(unaudited)
Net asset value, beginning of period		$34.08			$29.02			$27.62			$27.23			$24.74			$18.40
Income (loss) from investment operations
Net investment income (loss) (d)		$0.01			$(0.10	)(c)		$(0.03	)		$(0.10	)		$(0.12	)		$0.04
Net realized and unrealized gain (loss)		(0.32	)		5.43			1.43			0.49			2.61			6.38
Total from investment operations		$(0.31	)		$5.33			$1.40			$0.39			$2.49			$6.42
Less distributions declared to shareholders
From net investment income		$—			$—			$—			$—			$—			$(0.08	)
From net realized gain		(2.97	)		(0.27	)		—			—			—			—
Total distributions declared to shareholders		$(2.97	)		$(0.27	)		$—			$—			$—			$(0.08	)
Net asset value, end of period (x)		$30.80			$34.08			$29.02			$27.62			$27.23			$24.74
Total return (%) (r)(s)(t)(x)		(1.16	)(n)		18.48	(c)		5.07			1.43			10.06			35.07
Ratios (%) (to average net assets) and Supplemental data:
Expenses before expense reductions (f)		1.94	(a)		1.94	(c)		1.95			1.96			1.95			2.03
Expenses after expense reductions (f)		1.93	(a)		1.94	(c)		1.95			1.95			1.94			2.03
Net investment income (loss)		0.07	(a)		(0.33	)(c)		(0.12	)		(0.36	)		(0.45	)		0.20
Portfolio turnover		57	(n)		121			109			119			106			115
Net assets at end of period (000 omitted)		$74,751			$95,208			$105,432			$108,122			$106,272			$49,276

See Notes to Financial Statements

14

Financial Highlights – continued

	Six months ended 4/30/18			Year ended
Class I				10/31/17			10/31/16			10/31/15			10/31/14			10/31/13
	(unaudited)
Net asset value, beginning of period		$35.88			$30.49			$28.88			$28.35			$25.61			$19.05
Income (loss) from investment operations
Net investment income (loss) (d)		$0.18			$0.22	(c)		$0.26			$0.19			$0.15			$0.23
Net realized and unrealized gain (loss)		(0.34	)		5.70			1.51			0.49			2.71			6.61
Total from investment operations		$(0.16	)		$5.92			$1.77			$0.68			$2.86			$6.84
Less distributions declared to shareholders
From net investment income		$(0.18	)		$(0.26	)		$(0.16	)		$(0.15	)		$(0.12	)		$(0.28	)
From net realized gain		(2.97	)		(0.27	)		—			—			—			—
Total distributions declared to shareholders		$(3.15	)		$(0.53	)		$(0.16	)		$(0.15	)		$(0.12	)		$(0.28	)
Net asset value, end of period (x)		$32.57			$35.88			$30.49			$28.88			$28.35			$25.61
Total return (%) (r)(s)(t)(x)		(0.68	)(n)		19.64	(c)		6.17			2.42			11.19			36.40
Ratios (%) (to average net assets) and Supplemental data:
Expenses before expense reductions (f)		0.94	(a)		0.95	(c)		0.95			0.96			0.95			1.03
Expenses after expense reductions (f)		0.93	(a)		0.94	(c)		0.95			0.95			0.95			1.02
Net investment income (loss)		1.07	(a)		0.65	(c)		0.88			0.66			0.54			1.01
Portfolio turnover		57	(n)		121			109			119			106			115
Net assets at end of period (000 omitted)		$131,849			$156,864			$112,860			$110,990			$146,339			$56,117

See Notes to Financial Statements

15

Financial Highlights – continued

	Six months ended 4/30/18			Year ended
Class R1				10/31/17			10/31/16			10/31/15			10/31/14			10/31/13
	(unaudited)
Net asset value, beginning of period		$33.97			$28.93			$27.53			$27.14			$24.66			$18.36
Income (loss) from investment operations
Net investment income (loss) (d)		$0.01			$(0.11	)(c)		$(0.02	)		$(0.10	)		$(0.13	)		$(0.00	)(w)
Net realized and unrealized gain (loss)		(0.33	)		5.42			1.42			0.49			2.61			6.40
Total from investment operations		$(0.32	)		$5.31			$1.40			$0.39			$2.48			$6.40
Less distributions declared to shareholders
From net investment income		$—			$—			$—			$—			$—			$(0.10	)
From net realized gain		(2.97	)		(0.27	)		—			—			—			—
Total distributions declared to shareholders		$(2.97	)		$(0.27	)		$—			$—			$—			$(0.10	)
Net asset value, end of period (x)		$30.68			$33.97			$28.93			$27.53			$27.14			$24.66
Total return (%) (r)(s)(t)(x)		(1.19	)(n)		18.47	(c)		5.09			1.44			10.06			35.04
Ratios (%) (to average net assets) and Supplemental data:
Expenses before expense reductions (f)		1.94	(a)		1.95	(c)		1.95			1.96			1.95			2.02
Expenses after expense reductions (f)		1.93	(a)		1.94	(c)		1.95			1.96			1.95			2.02
Net investment income (loss)		0.07	(a)		(0.34	)(c)		(0.08	)		(0.37	)		(0.48	)		(0.01	)
Portfolio turnover		57	(n)		121			109			119			106			115
Net assets at end of period (000 omitted)		$222			$310			$258			$1,643			$1,397			$418

See Notes to Financial Statements

16

Financial Highlights – continued

	Six months ended 4/30/18			Year ended
Class R2				10/31/17			10/31/16		10/31/15			10/31/14			10/31/13
	(unaudited)
Net asset value, beginning of period		$34.78			$29.58			$28.00		$27.59			$24.94			$18.56
Income (loss) from investment operations
Net investment income (loss) (d)		$0.10			$0.06	(c)		$0.11		$0.04			$(0.02	)		$0.15
Net realized and unrealized gain (loss)		(0.34	)		5.53			1.47		0.48			2.68			6.42
Total from investment operations		$(0.24	)		$5.59			$1.58		$0.52			$2.66			$6.57
Less distributions declared to shareholders
From net investment income		$—			$(0.12	)		$—		$(0.11	)		$(0.01	)		$(0.19	)
From net realized gain		(2.97	)		(0.27	)		—		—			—			—
Total distributions declared to shareholders		$(2.97	)		$(0.39	)		$—		$(0.11	)		$(0.01	)		$(0.19	)
Net asset value, end of period (x)		$31.57			$34.78			$29.58		$28.00			$27.59			$24.94
Total return (%) (r)(s)(t)(x)		(0.92	)(n)		19.05	(c)		5.64		1.89			10.66			35.75
Ratios (%) (to average net assets) and Supplemental data:
Expenses before expense reductions (f)		1.44	(a)		1.45	(c)		1.45		1.46			1.48			1.53
Expenses after expense reductions (f)		1.43	(a)		1.44	(c)		1.45		1.45			1.47			1.53
Net investment income (loss)		0.58	(a)		0.20	(c)		0.38		0.14			(0.09	)		0.71
Portfolio turnover		57	(n)		121			109		119			106			115
Net assets at end of period (000 omitted)		$643			$1,270			$1,716		$1,894			$1,728			$185

See Notes to Financial Statements

17

Financial Highlights – continued

	Six months ended 4/30/18			Year ended
Class R3				10/31/17			10/31/16			10/31/15			10/31/14			10/31/13
	(unaudited)
Net asset value, beginning of period		$35.64			$30.28			$28.68			$28.19			$25.49			$18.96
Income (loss) from investment operations
Net investment income (loss) (d)		$0.14			$0.15	(c)		$0.18			$0.11			$0.07			$0.16
Net realized and unrealized gain (loss)		(0.35	)		5.66			1.50			0.49			2.70			6.60
Total from investment operations		$(0.21	)		$5.81			$1.68			$0.60			$2.77			$6.76
Less distributions declared to shareholders
From net investment income		$(0.08	)		$(0.18	)		$(0.08	)		$(0.11	)		$(0.07	)		$(0.23	)
From net realized gain		(2.97	)		(0.27	)		—			—			—			—
Total distributions declared to shareholders		$(3.05	)		$(0.45	)		$(0.08	)		$(0.11	)		$(0.07	)		$(0.23	)
Net asset value, end of period (x)		$32.38			$35.64			$30.28			$28.68			$28.19			$25.49
Total return (%) (r)(s)(t)(x)		(0.83	)(n)		19.39	(c)		5.87			2.17			10.90			36.09
Ratios (%) (to average net assets) and Supplemental data:
Expenses before expense reductions (f)		1.19	(a)		1.19	(c)		1.20			1.21			1.20			1.27
Expenses after expense reductions (f)		1.18	(a)		1.19	(c)		1.20			1.20			1.20			1.27
Net investment income (loss)		0.81	(a)		0.47	(c)		0.63			0.38			0.26			0.69
Portfolio turnover		57	(n)		121			109			119			106			115
Net assets at end of period (000 omitted)		$1,121			$1,210			$1,593			$1,677			$1,492			$425

See Notes to Financial Statements

18

Financial Highlights – continued

	Six months ended 4/30/18			Year ended
Class R4				10/31/17			10/31/16			10/31/15			10/31/14			10/31/13
	(unaudited)
Net asset value, beginning of period		$35.93			$30.52			$28.90			$28.37			$25.62			$19.05
Income (loss) from investment operations
Net investment income (loss) (d)		$0.22			$0.23	(c)		$0.26			$0.18			$0.16			$0.27
Net realized and unrealized gain (loss)		(0.37	)		5.71			1.50			0.49			2.69			6.58
Total from investment operations		$(0.15	)		$5.94			$1.76			$0.67			$2.85			$6.85
Less distributions declared to shareholders
From net investment income		$(0.15	)		$(0.26	)		$(0.14	)		$(0.14	)		$(0.10	)		$(0.28	)
From net realized gain		(2.97	)		(0.27	)		—			—			—			—
Total distributions declared to shareholders		$(3.12	)		$(0.53	)		$(0.14	)		$(0.14	)		$(0.10	)		$(0.28	)
Net asset value, end of period (x)		$32.66			$35.93			$30.52			$28.90			$28.37			$25.62
Total return (%) (r)(s)(t)(x)		(0.63	)(n)		19.68	(c)		6.11			2.41			11.18			36.45
Ratios (%) (to average net assets) and Supplemental data:
Expenses before expense reductions (f)		0.94	(a)		0.95	(c)		0.95			0.96			0.94			1.03
Expenses after expense reductions (f)		0.93	(a)		0.94	(c)		0.95			0.96			0.94			1.03
Net investment income (loss)		1.25	(a)		0.70	(c)		0.87			0.61			0.57			1.23
Portfolio turnover		57	(n)		121			109			119			106			115
Net assets at end of period (000 omitted)		$607			$4,199			$6,656			$6,322			$9,664			$6,535

See Notes to Financial Statements

19

Financial Highlights – continued

	Six months ended 4/30/18			Year ended
Class R6				10/31/17			10/31/16			10/31/15			10/31/14			10/31/13
	(unaudited)
Net asset value, beginning of period		$36.48			$30.99			$29.36			$28.80			$26.02			$19.07
Income (loss) from investment operations
Net investment income (loss) (d)		$0.20			$0.25	(c)		$0.29			$0.23			$0.22			$0.19
Net realized and unrealized gain (loss)		(0.34	)		5.80			1.53			0.50			2.68			6.78
Total from investment operations		$(0.14	)		$6.05			$1.82			$0.73			$2.90			$6.97
Less distributions declared to shareholders
From net investment income		$(0.21	)		$(0.29	)		$(0.19	)		$(0.17	)		$(0.12	)		$(0.02	)
From net realized gain		(2.97	)		(0.27	)		—			—			—			—
Total distributions declared to shareholders		$(3.18	)		$(0.56	)		$(0.19	)		$(0.17	)		$(0.12	)		$(0.02	)
Net asset value, end of period (x)		$33.16			$36.48			$30.99			$29.36			$28.80			$26.02
Total return (%) (r)(s)(t)(x)		(0.60	)(n)		19.75	(c)		6.25			2.58			11.20			36.60
Ratios (%) (to average net assets) and Supplemental data:
Expenses before expense reductions (f)		0.85	(a)		0.85	(c)		0.85			0.85			0.86			0.92
Expenses after expense reductions (f)		0.84	(a)		0.84	(c)		0.85			0.85			0.86			0.92
Net investment income (loss)		1.14	(a)		0.72	(c)		0.95			0.81			0.79			0.79
Portfolio turnover		57	(n)		121			109			119			106			115
Net assets at end of period (000 omitted)		$15,055			$16,373			$10,178			$7,115			$1,647			$4,399

See Notes to Financial Statements

20

Financial Highlights – continued

(a) Annualized.

(c) Amount reflects a one-time reimbursement of expenses by the custodian (or former custodian) without which net investment income and performance would be lower and expenses would be higher.

(d) Per share data is based on average shares outstanding.

(f) Ratios do not reflect reductions from fees paid indirectly, if applicable.

(n) Not annualized.

(r) Certain expenses have been reduced without which performance would have been lower.

(s) From time to time the fund may receive proceeds from litigation settlements, without which performance would be lower.

(t) Total returns do not include any applicable sales charges.

(w) Per share amount was less than $0.01.

(x) The net asset values and total returns have been calculated on net assets which include adjustments made in accordance with U.S. generally accepted accounting principles required at period end for financial reporting purposes.

See Notes to Financial Statements

21

NOTES TO FINANCIAL STATEMENTS

(unaudited)

(1) Business and Organization

MFS Equity Opportunities Fund (the fund) is a diversified series of MFS Series Trust XII (the trust). The trust is organized as a Massachusetts business trust and is registered under the Investment Company Act of 1940, as amended, as an open-end management investment company.

The fund is an investment company and accordingly follows the investment company accounting and reporting guidance of the Financial Accounting Standards Board (FASB) Accounting Standards Codification Topic 946 Financial Services – Investment Companies.

(2) Significant Accounting Policies

General – The preparation of financial statements in conformity with U.S. generally accepted accounting principles requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of increases and decreases in net assets from operations during the reporting period. Actual results could differ from those estimates. In the preparation of these financial statements, management has evaluated subsequent events occurring after the date of the fund’s Statement of Assets and Liabilities through the date that the financial statements were issued.

Balance Sheet Offsetting – The fund’s accounting policy with respect to balance sheet offsetting is that, absent an event of default by the counterparty or a termination of the agreement, the International Swaps and Derivatives Association (ISDA) Master Agreement, or similar agreement, does not result in an offset of reported amounts of financial assets and financial liabilities in the Statement of Assets and Liabilities across transactions between the fund and the applicable counterparty. The fund’s right to setoff may be restricted or prohibited by the bankruptcy or insolvency laws of the particular jurisdiction to which a specific master netting agreement counterparty is subject. Balance sheet offsetting disclosures, to the extent applicable to the fund, have been included in the fund’s Significant Accounting Policies note under the captions for each of the fund’s in-scope financial instruments and transactions.

Investment Valuations – Equity securities, including restricted equity securities, are generally valued at the last sale or official closing price on their primary market or exchange as provided by a third-party pricing service. Equity securities, for which there were no sales reported that day, are generally valued at the last quoted daily bid quotation on their primary market or exchange as provided by a third-party pricing service. Short-term instruments with a maturity at issuance of 60 days or less may be valued at amortized cost, which approximates market value. Open-end investment companies are generally valued at net asset value per share. Securities and other assets generally valued on the basis of information from a third-party pricing service may also be valued at a broker/dealer bid quotation. Values obtained from third-party pricing services can utilize both transaction data and market information such as yield, quality, coupon rate, maturity, type of issue, trading characteristics, and other market data.

The Board of Trustees has delegated primary responsibility for determining or causing to be determined the value of the fund’s investments (including any fair valuation) to

22

Notes to Financial Statements (unaudited) – continued

the adviser pursuant to valuation policies and procedures approved by the Board. If the adviser determines that reliable market quotations are not readily available, investments are valued at fair value as determined in good faith by the adviser in accordance with such procedures under the oversight of the Board of Trustees. Under the fund’s valuation policies and procedures, market quotations are not considered to be readily available for most types of debt instruments and floating rate loans and many types of derivatives. These investments are generally valued at fair value based on information from third-party pricing services. In addition, investments may be valued at fair value if the adviser determines that an investment’s value has been materially affected by events occurring after the close of the exchange or market on which the investment is principally traded (such as foreign exchange or market) and prior to the determination of the fund’s net asset value, or after the halting of trading of a specific security where trading does not resume prior to the close of the exchange or market on which the security is principally traded. Events that occur on a frequent basis after foreign markets close (such as developments in foreign markets and significant movements in the U.S. markets) and prior to the determination of the fund’s net asset value may be deemed to have a material effect on the value of securities traded in foreign markets. Accordingly, the fund’s foreign equity securities may often be valued at fair value. The adviser generally relies on third-party pricing services or other information (such as the correlation with price movements of similar securities in the same or other markets; the type, cost and investment characteristics of the security; the business and financial condition of the issuer; and trading and other market data) to assist in determining whether to fair value and at what value to fair value an investment. The value of an investment for purposes of calculating the fund’s net asset value can differ depending on the source and method used to determine value. When fair valuation is used, the value of an investment used to determine the fund’s net asset value may differ from quoted or published prices for the same investment. There can be no assurance that the fund could obtain the fair value assigned to an investment if it were to sell the investment at the same time at which the fund determines its net asset value per share.

Various inputs are used in determining the value of the fund’s assets or liabilities. These inputs are categorized into three broad levels. In certain cases, the inputs used to measure fair value may fall into different levels of the fair value hierarchy. In such cases, an investment’s level within the fair value hierarchy is based on the lowest level of input that is significant to the fair value measurement. The fund’s assessment of the significance of a particular input to the fair value measurement in its entirety requires judgment, and considers factors specific to the investment. Level 1 includes unadjusted quoted prices in active markets for identical assets or liabilities. Level 2 includes other significant observable market-based inputs (including quoted prices for similar securities, interest rates, prepayment speed, and credit risk). Level 3 includes unobservable inputs, which may include the adviser’s own assumptions in determining the fair value of investments. The following is a summary of the levels used as of April 30, 2018 in valuing the fund’s assets or liabilities:

Financial Instruments	Level 1		Level 2		Level 3		Total
Equity Securities		$399,925,348		$—		$—		$399,925,348
Mutual Funds		1,788,479		—		—		1,788,479
Total		$401,713,827		$—		$—		$401,713,827

23

Notes to Financial Statements (unaudited) – continued

For further information regarding security characteristics, see the Portfolio of Investments.

Indemnifications – Under the fund’s organizational documents, its officers and Trustees may be indemnified against certain liabilities and expenses arising out of the performance of their duties to the fund. Additionally, in the normal course of business, the fund enters into agreements with service providers that may contain indemnification clauses. The fund’s maximum exposure under these agreements is unknown as this would involve future claims that may be made against the fund that have not yet occurred.

Investment Transactions and Income – Investment transactions are recorded on the trade date. Dividends received in cash are recorded on the ex-dividend date. Certain dividends from foreign securities will be recorded when the fund is informed of the dividend if such information is obtained subsequent to the ex-dividend date. Dividend payments received in additional securities are recorded on the ex-dividend date in an amount equal to the value of the security on such date.

The fund may receive proceeds from litigation settlements. Any proceeds received from litigation involving portfolio holdings are reflected in the Statement of Operations in realized gain/loss if the security has been disposed of by the fund or in unrealized gain/loss if the security is still held by the fund. Any other proceeds from litigation not related to portfolio holdings are reflected as other income in the Statement of Operations.

Tax Matters and Distributions – The fund intends to qualify as a regulated investment company, as defined under Subchapter M of the Internal Revenue Code, and to distribute all of its taxable income, including realized capital gains. As a result, no provision for federal income tax is required. The fund’s federal tax returns, when filed, will remain subject to examination by the Internal Revenue Service for a three year period. Management has analyzed the fund’s tax positions taken on federal and state tax returns for all open tax years and does not believe that there are any uncertain tax positions that require recognition of a tax liability.

Distributions to shareholders are recorded on the ex-dividend date. Income and capital gain distributions are determined in accordance with income tax regulations, which may differ from U.S. generally accepted accounting principles. Certain capital accounts in the financial statements are periodically adjusted for permanent differences in order to reflect their tax character. These adjustments have no impact on net assets or net asset value per share. Temporary differences which arise from recognizing certain items of income, expense, gain or loss in different periods for financial statement and tax purposes will reverse at some time in the future. Distributions in excess of net investment income or net realized gains are temporary overdistributions for financial statement purposes resulting from differences in the recognition or classification of income or distributions for financial statement and tax purposes.

Book/tax differences primarily relate to wash sale loss deferrals, treating a portion of the proceeds from redemptions as a distribution for tax purposes, and partnerships.

24

Notes to Financial Statements (unaudited) – continued

The tax character of distributions made during the current period will be determined at fiscal year end. The tax character of distributions declared to shareholders for the last fiscal year is as follows:

	Year ended 10/31/17
Ordinary income (including any short-term capital gains)		$2,350,035
Long-term capital gains		4,101,140
Total distributions		$6,451,175

The federal tax cost and the tax basis components of distributable earnings were as follows:

As of 4/30/18
Cost of investments	$375,972,577
Gross appreciation	38,538,131
Gross depreciation	(12,796,881	)
Net unrealized appreciation (depreciation)	$25,741,250
As of 10/31/17
Undistributed ordinary income	7,974,016
Undistributed long-term capital gain	31,092,488
Other temporary differences	(382,089	)
Net unrealized appreciation (depreciation)	65,292,103

The aggregate cost above includes prior fiscal year end tax adjustments, if applicable.

Multiple Classes of Shares of Beneficial Interest – The fund offers multiple classes of shares, which differ in their respective distribution and service fees. The fund’s income, realized and unrealized gain (loss), and common expenses are allocated to shareholders based on the daily net assets of each class. Dividends are declared separately for each class. Differences in per share dividend rates are generally due to differences in separate class expenses. Class B shares will convert to Class A shares approximately eight years after purchase. Effective April 23, 2018, Class C shares will

25

Notes to Financial Statements (unaudited) – continued

convert to Class A shares approximately ten years after purchase. The fund’s distributions declared to shareholders as reported in the Statements of Changes in Net Assets are presented by class as follows:

	From net investment income				From net realized gain
	Six months ended 4/30/18		Year ended 10/31/17		Six months ended 4/30/18		Year ended 10/31/17
Class A		$406,404		$1,203,331		$14,397,767		$1,824,850
Class B		—		—		1,102,773		126,354
Class C		—		—		8,085,856		950,683
Class I		756,202		975,525		12,756,323		1,016,269
Class R1		—		—		26,534		2,216
Class R2		—		6,730		108,819		15,650
Class R3		2,584		9,647		102,175		14,270
Class R4		17,306		56,153		340,821		58,605
Class R6		92,578		98,649		1,311,099		92,243
Total		$1,275,074		$2,350,035		$38,232,167		$4,101,140

(3) Transactions with Affiliates

Investment Adviser – The fund has an investment advisory agreement with MFS to provide overall investment management and related administrative services and facilities to the fund. The management fee is computed daily and paid monthly at the following annual rates based on the fund’s average daily net assets:

Up to $1 billion	0.75	%
In excess of $1 billion and up to $2.5 billion	0.65	%
In excess of $2.5 billion and up to $5 billion	0.60	%
In excess of $5 billion	0.50	%

MFS has agreed in writing to reduce its management fee by a specified amount if certain MFS mutual fund assets exceed thresholds agreed to by MFS and the fund’s Board of Trustees. For the six months ended April 30, 2018, this management fee reduction amounted to $19,725, which is included in the reduction of total expenses in the Statement of Operations. The management fee incurred for the six months ended April 30, 2018 was equivalent to an annual effective rate of 0.74% of the fund’s average daily net assets.

The investment adviser has agreed in writing to pay a portion of the fund’s total annual operating expenses, excluding interest, taxes, extraordinary expenses, brokerage and transaction costs, and investment-related expenses, such that total fund operating expenses do not exceed the following rates annually of each class’s average daily net assets:

Classes
A	B		C		I		R1		R2		R3		R4		R6
1.40%		2.15%		2.15%		1.15%		2.15%		1.65%		1.40%		1.15%		1.09%

This written agreement will continue until modified by the fund’s Board of Trustees, but such agreement will continue at least until February 28, 2019. For the six months

26

Notes to Financial Statements (unaudited) – continued

ended April 30, 2018, the fund’s actual operating expenses did not exceed the limit and therefore, the investment adviser did not pay any portion of the fund’s expenses related to this agreement.

Distributor – MFS Fund Distributors, Inc. (MFD), a wholly-owned subsidiary of MFS, as distributor, received $11,233 for the six months ended April 30, 2018, as its portion of the initial sales charge on sales of Class A shares of the fund.

The Board of Trustees has adopted a distribution plan for certain share classes pursuant to Rule 12b-1 of the Investment Company Act of 1940.

The fund’s distribution plan provides that the fund will pay MFD for services provided by MFD and financial intermediaries in connection with the distribution and servicing of certain share classes. One component of the plan is a distribution fee paid to MFD and another component of the plan is a service fee paid to MFD. MFD may subsequently pay all, or a portion, of the distribution and/or service fees to financial intermediaries.

Distribution Plan Fee Table:

	Distribution Fee Rate (d)		Service Fee Rate (d)		Total Distribution Plan (d)		Annual Effective Rate (e)		Distribution and Service Fee
Class A		—		0.25%		0.25%		0.24%		$212,752
Class B		0.75%		0.25%		1.00%		1.00%		60,132
Class C		0.75%		0.25%		1.00%		1.00%		453,243
Class R1		0.75%		0.25%		1.00%		1.00%		1,453
Class R2		0.25%		0.25%		0.50%		0.50%		2,544
Class R3		—		0.25%		0.25%		0.25%		1,492
Total Distribution and Service Fees										$731,616

(d) In accordance with the distribution plan for certain classes, the fund pays distribution and/or service fees equal to these annual percentage rates of each class’s average daily net assets. The distribution and service fee rates disclosed by class represent the current rates in effect at the end of the reporting period. Any rate changes, if applicable, are detailed below.

(e) The annual effective rates represent actual fees incurred under the distribution plan for the six months ended April 30, 2018 based on each class’s average daily net assets. MFD has voluntarily agreed to rebate a portion of each class’s 0.25% service fee attributable to accounts for which MFD retains the 0.25% service fee except for accounts attributable to MFS or its affiliates’ seed money. For the six months ended April 30, 2018, this rebate amounted to $4,932, $26, $328, and $16 for Class A, Class B, Class C, and Class R3, respectively, and is included in the reduction of total expenses in the Statement of Operations.

Certain Class A shares are subject to a contingent deferred sales charge (CDSC) in the event of a shareholder redemption within 18 months of purchase. Class B shares are subject to a CDSC in the event of a shareholder redemption within six years of

purchase. Class C shares are subject to a CDSC in the event of a shareholder redemption within 12 months of purchase. All contingent deferred sales charges are paid to MFD and during the six months ended April 30, 2018, were as follows:

	Amount
Class A		$991
Class B		16,613
Class C		2,548

27

Notes to Financial Statements (unaudited) – continued

Shareholder Servicing Agent – MFS Service Center, Inc. (MFSC), a wholly-owned subsidiary of MFS, receives a fee from the fund for its services as shareholder servicing agent calculated as a percentage of the average daily net assets of the fund as determined periodically under the supervision of the fund’s Board of Trustees. For the six months ended April 30, 2018, the fee was $26,966, which equated to 0.0121% annually of the fund’s average daily net assets. MFSC also receives payment from the fund for out-of-pocket expenses, sub-accounting and other shareholder servicing costs which may be paid to affiliated and unaffiliated service providers. Class R6 shares do not incur sub-accounting fees. For the six months ended April 30, 2018, these out-of-pocket expenses, sub-accounting and other shareholder servicing costs amounted to $211,705.

Administrator – MFS provides certain financial, legal, shareholder communications, compliance, and other administrative services to the fund. Under an administrative services agreement, the fund reimburses MFS the costs incurred to provide these services. The fund is charged an annual fixed amount of $17,500 plus a fee based on average daily net assets. The administrative services fee incurred for the six months ended April 30, 2018 was equivalent to an annual effective rate of 0.0171% of the fund’s average daily net assets.

Trustees’ and Officers’ Compensation – The fund pays compensation to independent Trustees in the form of a retainer, attendance fees, and additional compensation to Board and Committee chairpersons. The fund does not pay compensation directly to Trustees or officers of the fund who are also officers of the investment adviser, all of whom receive remuneration for their services to the fund from MFS. Certain officers and Trustees of the fund are officers or directors of MFS, MFD, and MFSC.

Other – This fund and certain other funds managed by MFS (the funds) have entered into a service agreement (the ISO Agreement) which provides for payment of fees solely by the funds to Tarantino LLC in return for the provision of services of an Independent Senior Officer (ISO) for the funds. Frank L. Tarantino serves as the ISO and is an officer of the funds and the sole member of Tarantino LLC. The funds can terminate the ISO Agreement with Tarantino LLC at any time under the terms of the ISO Agreement. For the six months ended April 30, 2018, the fee paid by the fund under this agreement was $371 and is included in “Miscellaneous” expense in the Statement of Operations. MFS has agreed to bear all expenses associated with office space, other administrative support, and supplies provided to the ISO.

The fund invests in the MFS Institutional Money Market Portfolio which is managed by MFS and seeks current income consistent with preservation of capital and liquidity. This money market fund does not pay a management fee to MFS.

The fund is permitted to engage in purchase and sale transactions with funds and accounts for which MFS serves as investment adviser or sub-adviser (“cross-trades”) pursuant to a policy adopted by the Board of Trustees. This policy has been designed to ensure that cross-trades conducted by the fund comply with Rule 17a-7 under the Investment Company Act of 1940. During the six months ended April 30, 2018, the fund engaged in purchase and sale transactions pursuant to this policy, which amounted to $12,101,896 and $1,055,753, respectively. The sales transactions resulted in net realized gains (losses) of $(51,238).

28

Notes to Financial Statements (unaudited) – continued

Effective January 3, 2018, the adviser has voluntarily undertaken to reimburse the fund from its own resources on a quarterly basis for the cost of investment research embedded in the cost of the fund’s securities trades. This agreement may be rescinded at any time. This reimbursement, if any, will be determined in arrears on a quarterly basis beginning with the quarter ending March 31, 2018.

(4) Portfolio Securities

For the six months ended April 30, 2018, purchases and sales of investments, other than short-term obligations, aggregated $252,459,378 and $310,857,230, respectively.

(5) Shares of Beneficial Interest

The fund’s Declaration of Trust permits the Trustees to issue an unlimited number of full and fractional shares of beneficial interest. Transactions in fund shares were as follows:

	Six months ended 4/30/18				Year ended 10/31/17
	Shares		Amount		Shares		Amount
Shares sold
Class A		437,322		$14,582,081		584,839		$19,179,664
Class B		8,058		261,842		37,035		1,150,982
Class C		88,330		2,865,141		257,289		8,012,759
Class I		284,873		9,782,437		2,486,597		81,255,585
Class R1		596		19,135		2,654		82,551
Class R2		1,224		40,494		8,671		280,324
Class R3		4,186		146,494		6,859		226,875
Class R4		1,665		56,311		114,609		3,797,746
Class R6		69,843		2,427,963		198,628		6,856,572
		896,097		$30,181,898		3,697,181		$120,843,058
Shares issued to shareholders in reinvestment of distributions
Class A		422,296		$14,083,559		91,968		$2,901,605
Class B		33,665		1,065,824		4,021		121,593
Class C		227,960		7,224,055		25,835		782,012
Class I		342,626		11,440,296		51,422		1,624,405
Class R1		840		26,534		73		2,216
Class R2		3,355		108,819		728		22,380
Class R3		3,153		104,759		761		23,917
Class R4		10,700		358,127		1,038		32,837
Class R6		21,092		716,480		1,716		55,076
		1,065,687		$35,128,453		177,562		$5,566,041

29

Notes to Financial Statements (unaudited) – continued

	Six months ended 4/30/18						Year ended 10/31/17
	Shares			Amount			Shares			Amount
Shares reacquired
Class A		(702,769	)		$(23,872,788	)		(2,669,102	)		$(87,393,737	)
Class B		(71,177	)		(2,297,747	)		(139,151	)		(4,392,146	)
Class C		(682,653	)		(21,786,489	)		(1,122,409	)		(35,513,392	)
Class I		(950,977	)		(32,148,756	)		(1,868,446	)		(62,240,670	)
Class R1		(3,313	)		(103,292	)		(2,535	)		(80,282	)
Class R2		(20,718	)		(679,636	)		(30,912	)		(1,007,093	)
Class R3		(6,669	)		(227,011	)		(26,296	)		(867,675	)
Class R4		(110,652	)		(3,713,085	)		(216,819	)		(7,420,195	)
Class R6		(85,713	)		(3,022,457	)		(79,986	)		(2,717,547	)
		(2,634,641	)		$(87,851,261	)		(6,155,656	)		$(201,632,737	)
Net change
Class A		156,849			$4,792,852			(1,992,295	)		$(65,312,468	)
Class B		(29,454	)		(970,081	)		(98,095	)		(3,119,571	)
Class C		(366,363	)		(11,697,293	)		(839,285	)		(26,718,621	)
Class I		(323,478	)		(10,926,023	)		669,573			20,639,320
Class R1		(1,877	)		(57,623	)		192			4,485
Class R2		(16,139	)		(530,323	)		(21,513	)		(704,389	)
Class R3		670			24,242			(18,676	)		(616,883	)
Class R4		(98,287	)		(3,298,647	)		(101,172	)		(3,589,612	)
Class R6		5,222			121,986			120,358			4,194,101
		(672,857	)		$(22,540,910	)		(2,280,913	)		$(75,223,638	)

Class T shares were not publicly available for sale during the period. Please see the fund’s prospectus for details.

(6) Line of Credit

The fund and certain other funds managed by MFS participate in a $1.25 billion unsecured committed line of credit, subject to a $1 billion sublimit, provided by a syndication of banks under a credit agreement. Borrowings may be made for temporary financing needs. Interest is charged to each fund, based on its borrowings, generally at a rate equal to the higher of the Overnight Federal Reserve funds rate or daily one month LIBOR plus an agreed upon spread. A commitment fee, based on the average daily, unused portion of the committed line of credit, is allocated among the participating funds at the end of each calendar quarter. In addition, the fund and other funds managed by MFS have established unsecured uncommitted borrowing arrangements with certain banks for temporary financing needs. Interest is charged to each fund, based on its borrowings, at a rate equal to the Overnight Federal Reserve funds rate plus an agreed upon spread. For the six months ended April 30, 2018, the fund’s commitment fee and interest expense were $1,503 and $0, respectively, and are included in “Miscellaneous” expense in the Statement of Operations.

30

Notes to Financial Statements (unaudited) – continued

(7) Investments in Affiliated Issuers

An affiliated issuer may be considered one in which the fund owns 5% or more of the outstanding voting securities, or a company which is under common control. For the purposes of this report, the fund assumes the following to be affiliated issuers:

Affiliated Issuers				Beginning Shares/Par Amount		Acquisitions Shares/Par Amount		Dispositions Shares/Par Amount			Ending Shares/Par Amount
MFS Institutional Money Market Portfolio					5,228,337		51,610,955		(55,050,634	)		1,788,658
Affiliated Issuers	Realized Gain (Loss)			Change in Unrealized Appreciation (Depreciation)		Capital Gain Distributions		Dividend Income			Ending Value
MFS Institutional Money Market Portfolio		$(697	)		$—		$—		$22,169			$1,788,479

31

PROXY VOTING POLICIES AND INFORMATION

MFS votes proxies on behalf of the fund pursuant to proxy voting policies and procedures that are available without charge, upon request, by calling 1-800-225-2606, by visiting mfs.com/proxyvoting, or by visiting the SEC’s Web site at http://www.sec.gov.

Information regarding how the fund voted proxies relating to portfolio securities during the most recent twelve-month period ended June 30 is available by August 31 of each year without charge by visiting mfs.com/proxyvoting, or by visiting the SEC’s Web site at http://www.sec.gov.

QUARTERLY PORTFOLIO DISCLOSURE

The fund will file a complete schedule of portfolio holdings with the Securities and Exchange Commission (the Commission) for the first and third quarters of each fiscal year on Form N-Q. A shareholder can obtain the quarterly portfolio holdings report at mfs.com. The fund’s Form N-Q is also available on the EDGAR database on the Commission’s Internet Web site at http://www.sec.gov, and may be reviewed and copied at the:

Public Reference Room

Securities and Exchange Commission

100 F Street, NE, Room 1580

Washington, D.C. 20549

Information on the operation of the Public Reference Room may be obtained by calling the Commission at 1-800-SEC-0330. Copies of the fund’s Form N-Q also may be obtained, upon payment of a duplicating fee, by electronic request at the following e-mail address: publicinfo@sec.gov or by writing the Public Reference Section at the above address.

FURTHER INFORMATION

From time to time, MFS may post important information about the fund or the MFS funds on the MFS web site (mfs.com). This information is available at https://www.mfs.com/en-us/what-we-do/announcements.html or at mfs.com/openendfunds by choosing the fund’s name.

32

INFORMATION ABOUT FUND CONTRACTS AND LEGAL CLAIMS

The fund has entered into contractual arrangements with an investment adviser, administrator, distributor, shareholder servicing agent, 529 program manager (if applicable), and custodian who each provide services to the fund. Unless expressly stated otherwise, shareholders are not parties to, or intended beneficiaries of these contractual arrangements, and these contractual arrangements are not intended to create any shareholder right to enforce them against the service providers or to seek any remedy under them against the service providers, either directly or on behalf of the fund.

Under the Trust’s By-Laws and Declaration of Trust, any claims asserted against or on behalf of the MFS Funds, including claims against Trustees and Officers, must be brought in state and federal courts located within the Commonwealth of Massachusetts.

PROVISION OF FINANCIAL REPORTS AND SUMMARY PROSPECTUSES

The fund produces financial reports every six months and updates its summary prospectus and prospectus annually. To avoid sending duplicate copies of materials to households, only one copy of the fund’s annual and semiannual report and summary prospectus may be mailed to shareholders having the same last name and residential address on the fund’s records. However, any shareholder may contact MFSC (please see back cover for address and telephone number) to request that copies of these reports and summary prospectuses be sent personally to that shareholder.

33

Save paper with eDelivery.

MFS® will send you prospectuses,

reports, and proxies directly via e-mail so you will get information faster with less mailbox clutter.

To sign up:

1. Go to mfs.com.

2. Log in via MFS® Access.

3. Select eDelivery.

If you own your MFS fund shares through a financial institution or a retirement plan, MFS® TALK, MFS® Access, or eDelivery may not be available to you.

CONTACT

WEB SITE

mfs.com

MFS TALK

1-800-637-8255

24 hours a day

ACCOUNT SERVICE AND LITERATURE

Shareholders

1-800-225-2606

Financial advisors

1-800-343-2829

Retirement plan services

1-800-637-1255

MAILING ADDRESS

MFS Service Center, Inc.

P.O. Box 55824

Boston, MA 02205-5824

OVERNIGHT MAIL

MFS Service Center, Inc.

c/o DST Asset Manager Solutions, Inc.

30 Dan Road

Canton, MA 02021-2809

ITEM 2. CODE OF ETHICS.

During the period covered by this report, the Registrant has not amended any provision in its Code of Ethics (the “Code”) that relates to an element of the Code’s definitions enumerated in paragraph (b) of Item 2 of this Form N-CSR. During the period covered by this report, the Registrant did not grant a waiver, including an implicit waiver, from any provision of the Code.

ITEM 3. AUDIT COMMITTEE FINANCIAL EXPERT.

Not applicable for semi-annual reports.

ITEM 4. PRINCIPAL ACCOUNTANT FEES AND SERVICES.

Not applicable for semi-annual reports.

ITEM 5. AUDIT COMMITTEE OF LISTED REGISTRANTS.

Not applicable to the Registrant.

ITEM 6. INVESTMENTS

A schedule of investments for MFS Equity Opportunities Fund is included as part of the report to shareholders under Item 1 of this Form N-CSR.

ITEM 7. DISCLOSURE OF PROXY VOTING POLICIES AND PROCEDURES FOR CLOSED-END MANAGEMENT INVESTMENT COMPANIES.

Not applicable to the Registrant.

ITEM 8. PORTFOLIO MANAGERS OF CLOSED-END MANAGEMENT INVESTMENT COMPANIES.

Not applicable to the Registrant.

ITEM 9. PURCHASES OF EQUITY SECURITIES BY CLOSED-END MANAGEMENT INVESTMENT COMPANY AND AFFILIATED PURCHASERS.

Not applicable to the Registrant.

ITEM 10. SUBMISSION OF MATTERS TO A VOTE OF SECURITY HOLDERS.

There were no material changes to the procedures by which shareholders may send recommendations to the Board for nominees to the Registrant’s Board since the Registrant last provided disclosure as to such procedures in response to the requirements of Item 407 (c)(2)(iv) of Regulation S-K or this Item.

ITEM 11. CONTROLS AND PROCEDURES.

(a) Based upon their evaluation of the effectiveness of the registrant’s disclosure controls and procedures (as defined in Rule 30a-3(c) under the Investment Company Act of 1940 (the “Act”)) as conducted within 90 days of the filing date of this report on Form N-CSR, the registrant’s principal financial officer and principal executive officer have concluded that those disclosure controls and procedures provide reasonable assurance that the material information required to be disclosed by the registrant on this report is recorded, processed, summarized and reported within the time periods specified in the Securities and Exchange Commission’s rules and forms.

(b) There were no changes in the registrant’s internal controls over financial reporting (as defined in Rule 30a-3(d) under the Act) that occurred during the second fiscal quarter of the period covered by the report that have materially affected, or are reasonably likely to materially affect, the registrant’s internal control over financial reporting.

ITEM 12. DISCLOSURE OF SECURITIES LENDING ACTIVITIES FOR CLOSED-END MANAGEMENT INVESTMENT COMPANIES.

Not applicable to the Registrant.

ITEM 13. EXHIBITS.

(a) File the exhibits listed below as part of this form. Letter or number the exhibits in the sequence indicated.

(1) Any code of ethics, or amendment thereto, that is the subject of the disclosure required by Item 2, to the extent that the registrant intends to satisfy the Item 2 requirements through filing of an exhibit.

(2) A separate certification for each principal executive officer and principal financial officer of the registrant as required by Rule 30a-2(a) under the Act (17 CFR 270.30a-2): Attached hereto.

(b) If the report is filed under Section 13(a) or 15(d) of the Exchange Act, provide the certifications required by Rule 30a-2(b) under the Act (17 CFR 270.30a-2(b)), Rule 13a-14(b) or Rule 15d-14(b) under the Exchange Act (17 CFR 240.13a-14(b) or 240.15d-14(b)) and Section 1350 of Chapter 63 of Title 18 of the United States Code (18 U.S.C. 1350) as an exhibit. A certification furnished pursuant to this paragraph will not be deemed “filed” for the purposes of Section 18 of the Exchange Act (15 U.S.C. 78r), or otherwise subject to the liability of that section. Such certification will not be deemed to be incorporated by reference into any filing under the Securities Act of 1933 or the Exchange Act, except to the extent that the registrant specifically incorporates it by reference: Attached hereto.

Notice

A copy of the Amended and Restated Declaration of Trust, as amended, of the Registrant is on file with the Secretary of State of The Commonwealth of Massachusetts and notice is hereby given that this instrument is executed on behalf of the Registrant by an officer of the Registrant as an officer and not individually and the obligations of or arising out of this instrument are not binding upon any of the Trustees or shareholders individually, but are binding only upon the assets and property of the respective constituent series of the Registrant.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934 and the Investment Company Act of 1940, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

(Registrant) MFS SERIES TRUST XII

By (Signature and Title)*	DAVID L. DILORENZO
	David L. DiLorenzo, President

Date: June 18, 2018

Pursuant to the requirements of the Securities Exchange Act of 1934 and the Investment Company Act of 1940, this report has been signed below by the following persons on behalf of the registrant and in the capacities and on the dates indicated.

By (Signature and Title)*	DAVID L. DILORENZO
	David L. DiLorenzo, President (Principal Executive Officer)

Date: June 18, 2018

By (Signature and Title)*	JAMES O. YOST
	James O. Yost, Treasurer (Principal Financial Officer and Accounting Officer)

Date: June 18, 2018

* Print name and title of each signing officer under his or her signature.

EX-99.CERT

EX-99.302CERT

MFS SERIES TRUST XII

CERTIFICATION PURSUANT TO SECTION 302 OF THE SARBANES-OXLEY ACT

I, James O. Yost, certify that:

1. I have reviewed this report on Form N-CSR of MFS Series Trust XII;

2. Based on my knowledge, this report does not contain any untrue statement of a material fact or omit to state a material fact necessary to make the statements made, in light of the circumstances under which such statements were made, not misleading with respect to the period covered by this report;

3. Based on my knowledge, the financial statements, and other financial information included in this report, fairly present in all material respects the financial condition, results of operations, changes in net assets, and cash flows (if the financial statements are required to include a statement of cash flows) of the registrant as of, and for, the periods presented in this report;

4. The registrant’s other certifying officer and I are responsible for establishing and maintaining disclosure controls and procedures (as defined in Rule 30a-3(c) under the Investment Company Act of 1940) and internal control over financial reporting (as defined in Rule 30a-3(d) under the Investment Company Act of 1940) for the registrant and have:

a. Designed such disclosure controls and procedures, or caused such disclosure controls and procedures to be designed under our supervision, to ensure that material information relating to the registrant, including its consolidated subsidiaries, is made known to us by others within those entities, particularly during the period in which this report is being prepared;

b. Designed such internal control over financial reporting, or caused such internal control over financial reporting to be designed under our supervision, to provide reasonable assurance regarding the reliability of financial reporting and the preparation of financial statements for external purposes in accordance with generally accepted accounting principles;

c. Evaluated the effectiveness of the registrant’s disclosure controls and procedures and presented in this report our conclusions about the effectiveness of the disclosure controls and procedures as of a date within 90 days prior to the filing date of this report based on such evaluation; and

d. Disclosed in this report any change in the registrant’s internal control over financial reporting that occurred during the second fiscal quarter of the period covered by the report that has materially affected, or is reasonably likely to materially affect, the registrant’s internal control over financial reporting; and

5. The registrant’s other certifying officer and I have disclosed to the registrant’s auditors and the audit committee of the registrant’s board of directors (or persons performing the equivalent functions):

a. All significant deficiencies and material weaknesses in the design or operation of internal control over financial reporting which are reasonably likely to adversely affect the registrant’s ability to record, process, summarize, and report financial information; and

b. Any fraud, whether or not material, that involves management or other employees who have a significant role in the registrant’s internal control over financial reporting.

Date: June 18, 2018	JAMES O. YOST
	James O. Yost
	Treasurer (Principal Financial Officer and Accounting Officer)

EX-99.302CERT

MFS SERIES TRUST XII

CERTIFICATION PURSUANT TO SECTION 302 OF THE SARBANES-OXLEY ACT

I, David L. DiLorenzo, certify that:

1. I have reviewed this report on Form N-CSR of MFS Series Trust XII;

2. Based on my knowledge, this report does not contain any untrue statement of a material fact or omit to state a material fact necessary to make the statements made, in light of the circumstances under which such statements were made, not misleading with respect to the period covered by this report;

3. Based on my knowledge, the financial statements, and other financial information included in this report, fairly present in all material respects the financial condition, results of operations, changes in net assets, and cash flows (if the financial statements are required to include a statement of cash flows) of the registrant as of, and for, the periods presented in this report;

4. The registrant’s other certifying officer and I are responsible for establishing and maintaining disclosure controls and procedures (as defined in Rule 30a-3(c) under the Investment Company Act of 1940) and internal control over financial reporting (as defined in Rule 30a-3(d) under the Investment Company Act of 1940) for the registrant and have:

a. Designed such disclosure controls and procedures, or caused such disclosure controls and procedures to be designed under our supervision, to ensure that material information relating to the registrant, including its consolidated subsidiaries, is made known to us by others within those entities, particularly during the period in which this report is being prepared;

b. Designed such internal control over financial reporting, or caused such internal control over financial reporting to be designed under our supervision, to provide reasonable assurance regarding the reliability of financial reporting and the preparation of financial statements for external purposes in accordance with generally accepted accounting principles;

c. Evaluated the effectiveness of the registrant’s disclosure controls and procedures and presented in this report our conclusions about the effectiveness of the disclosure controls and procedures as of a date within 90 days prior to the filing date of this report based on such evaluation; and

d. Disclosed in this report any change in the registrant’s internal control over financial reporting that occurred during the second fiscal quarter of the period covered by the report that has materially affected, or is reasonably likely to materially affect, the registrant’s internal control over financial reporting; and

5. The registrant’s other certifying officer and I have disclosed to the registrant’s auditors and the audit committee of the registrant’s board of directors (or persons performing the equivalent functions):

a. All significant deficiencies and material weaknesses in the design or operation of internal control over financial reporting which are reasonably likely to adversely affect the registrant’s ability to record, process, summarize, and report financial information; and

b. Any fraud, whether or not material, that involves management or other employees who have a significant role in the registrant’s internal control over financial reporting.

Date: June 18, 2018	DAVID L. DILORENZO
	David L. DiLorenzo
	President (Principal Executive Officer)

EX-99.906CERT

EX-99.906CERT

MFS SERIES TRUST XII

CERTIFICATION PURSUANT TO SECTION 906 OF THE SARBANES-OXLEY ACT

I, James O. Yost, certify that, to my knowledge:

1. The Form N-CSR (the “Report”) of MFS Series Trust XII (the “Registrant”) fully complies for the period covered by the Report with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934; and

2. The information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of the Registrant.

Date: June 18, 2018	JAMES O. YOST
	James O. Yost
	Treasurer (Principal Financial Officer and Accounting Officer)

A signed original of this written statement required by Section 906 has been provided to the Registrant and will be retained by the Registrant and furnished to the Securities and Exchange Commission or its staff upon request.

EX-99.906CERT

MFS SERIES TRUST XII

CERTIFICATION PURSUANT TO SECTION 906 OF THE SARBANES-OXLEY ACT

I, David L. DiLorenzo, certify that, to my knowledge:

1. The Form N-CSR (the “Report”) of MFS Series Trust XII (the “Registrant”) fully complies for the period covered by the Report with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934; and

2. The information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of the Registrant.

Date: June 18, 2018	DAVID L. DILORENZO
	David L. DiLorenzo
	President (Principal Executive Officer)

A signed original of this written statement required by Section 906 has been provided to the Registrant and will be retained by the Registrant and furnished to the Securities and Exchange Commission or its staff upon request.