CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 87,288	$ 10,315
Other receivables	1,011	304
Tax incentive receivables, current	1,932
Prepaid expenses and other current assets	1,828	1,253
Total current assets	92,059	11,872
Tax incentive receivables		144
Property and equipment, net	1,164	1,500
Deposits	206	206
Restricted cash	50	50
Total assets	93,479	13,772
Current liabilities:
Accounts payable	3,470	1,139
Accrued expenses and other current liabilities	4,321	2,928
Derivative liabilities	223	2,708
Deferred rent	143	143
Total current liabilities	8,157	6,918
Deferred rent, net of current portion	365	493
Total liabilities	8,522	7,411
Commitments and contingencies (Note 11)
Stockholders' equity (deficit):
Common units, zero and 335,281 units issued and outstanding as of December 31, 2017 and 2016, respectively	0	0
Preferred stock, $0.001 par value; 10,000,000 and zero shares authorized as of December 31, 2017 and 2016, respectively
Common stock, $0.001 par value; 60,000,000 shares authorized as of December 31, 3017; 14,369,182 shares issued and outstanding as of December 31, 2017; no shares authorized, issued or outstanding as of December 31, 2016	14
Additional paid-in capital	181,428
Accumulated deficit	(96,840)	(45,440)
Total Spero Therapeutics, Inc. stockholders' equity (deficit)	84,602	(45,440)
Non-controlling interests	355	(3,808)
Total stockholders' equity (deficit)	84,957	(49,248)
Total liabilities, redeemable convertible preferred units, and stockholders' equity (deficit)	$ 93,479	13,772
Bridge Units [Member]
Current liabilities:
Convertible preferred stock		7,924
Redeemable Convertible Preferred Units [Member]
Current liabilities:
Convertible preferred stock		$ 47,685