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EXHIBIT 99.1

ImClone Systems Incorporated
Investors:
Andrea F. Rabney
(646) 638-5058
Stefania Bethlen
(646) 638-5058 Media:
David M.F. Pitts
(646) 638-5058

For Immediate Release

IMCLONE SYSTEMS REPORTS FIRST QUARTER 2004 FINANCIAL RESULTS

Reports Significant Revenue and Profitability with Diluted Earnings Per Share of $0.76

New York, NY—April 27, 2004—ImClone Systems Incorporated (NASDAQ: IMCL) announced today its financial results for the quarter ended March 31, 2004.

        Total revenues for the first quarter of 2004 were $109.6 million as compared with $19.6 million for the first quarter of 2003. Included in first quarter 2004 revenues are four components: license fees and milestone revenues of $67.5 million; manufacturing revenue of $25.5 million; royalty revenue of $7.1 million; and collaborative agreement revenue of $9.5 million.

        Royalty revenue includes $6.8 million representing 39% of Bristol-Myers Squibb's in-market ERBITUX™ (Cetuximab) net sales of $17.5 million. These first quarter 2004 sales have been recorded from the commencement of distribution on February 24, 2004. These in-market sales, reflecting a drop-ship distribution methodology, represent ERBITUX shipments to end-user accounts only, with no wholesaler stocking.

        Total operating expenses for the first quarter of 2004 were $38.8 million as compared with $53.9 million for the first quarter of 2003. Research and development expenses for the first quarter of 2004 were $20.2 million as compared with $39.0 million for the first quarter of 2003. Clinical and regulatory expenses for the first quarter of 2004 were $7.1 million compared with $7.6 million for the first quarter of 2003. Marketing, general and administrative expenses for the first quarter of 2004 were $13.1 million as compared with $7.2 million for the first quarter of 2003. The effective tax rate has been estimated at 10 percent.

        Net income was $62.7 million with basic earnings per common share of $0.83 and diluted earnings per common share of $0.76 for the first quarter of 2004, compared with a net loss of $34.8 million or a loss per basic and diluted common share of $0.47 for the first quarter of 2003.

        "This announcement is very significant for ImClone Systems, as the February 12, 2004 FDA approval of ERBITUX to treat late-stage, metastatic colorectal cancer has allowed us to begin what we believe will be a period of continued profitability," stated Daniel S. Lynch, Chief Executive Officer of ImClone Systems. "We were pleased with the sales of ERBITUX in just over five weeks of product shipments during the quarter, as they appear to indicate that physicians are rapidly integrating this first-of-its-kind antibody into patients' treatment regimens."

        "The fact that the Company was able to achieve such an immediate and significant level of profitability in the first quarter of 2004 is reflective of several unique aspects of the income statement," said Michael J. Howerton, acting Chief Financial Officer of ImClone Systems. "These include the impact on revenues of the recognition of a portion of the $250 million milestone payment from Bristol-Myers Squibb, the absence of substantially all costs related to manufacturing revenue for product shipped during the period as these were expensed prior to product approval, and the expected utilization of significant net operating loss carryforwards and research and development credits."

Conference Call

        ImClone Systems will host a conference call with the financial community to discuss 2004 first quarter financial results, today, April 27, 2004 at 11:00 AM Eastern Time.

        The conference call will be webcast live and may be accessed by visiting ImClone Systems' website at www.imclone.com. A replay of the audio webcast will be available under "Earnings Webcast" in the "Investor Relations" section of the Company's website starting shortly after the call on April 27, 2004.

        Those parties interested in participating via telephone may join by dialing (888) 694-4641, or (973) 935-8512 for calls outside of Canada and the United States. A telephone replay of the conference call will be available shortly after the call until May 4, 2004 at midnight Eastern Time. To access the telephone replay, dial (877) 519-4471 domestically, or (973) 341-3080 for calls outside of Canada and the United States, and enter passcode number 4697335.

About ImClone Systems Incorporated

        ImClone Systems Incorporated is committed to advancing oncology care by developing and commercializing a portfolio of targeted biologic treatments designed to address the medical needs of patients with a variety of cancers. The Company's three programs include growth factor blockers, angiogenesis inhibitors and cancer vaccines. ImClone Systems' strategy is to become a fully integrated biopharmaceutical company, taking its development programs from the research stage to the market. ImClone Systems' headquarters and research operations are located in New York City, with additional administration and manufacturing facilities in Branchburg, New Jersey.

Certain matters discussed in this news release may constitute forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995 and the Federal securities laws. Although the company believes that the expectations reflected in such forward-looking statements are based upon reasonable assumptions, it can give no assurance that its expectations will be achieved. Forward-looking information is subject to certain risks, trends and uncertainties that could cause actual results to differ materially from those projected. Many of these factors are beyond the company's ability to control or predict. Important factors that may cause actual results to differ materially and could impact the company and the statements contained in this news release can be found in the company's filings with the Securities and Exchange Commission, including quarterly reports on Form 10-Q, current reports on Form 8-K and annual reports on Form 10-K. For forward-looking statements in this news release, the company claims the protection of the safe harbor for forward-looking statements contained in the Private Securities Litigation Reform Act of 1995. The company assumes no obligation to update or supplement any forward-looking statements, whether as a result of new information, future events or otherwise.

        (see attached tables)

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IMCLONE SYSTEMS INCORPORATED

Consolidated Statements of Income

(Unaudited)
(in thousands, except per share data)

				Three Months Ended March 31,
				2004			2003
Revenues:
	License fees and milestone revenues				67,498			10,622
	Manufacturing revenue				25,504			—
	Royalty revenue				7,145			303
	Collaborative agreement revenue				9,470			8,646
			Total revenues		109,617			19,571
Operating expenses:
	Research and development				20,211			39,040
	Clinical and regulatory				7,062			7,647
	Marketing, general and administrative				13,124			7,154
	Cost of manufacturing revenue				213			—
	Other (recoveries) expense				(1,815	)		33
		Total operating expenses			38,795			53,874
			Operating income (loss)		70,822			(34,303	)
Other expense, net					1,115			406
			Income (loss) before income taxes		69,707			(34,709	)
			Provision for income taxes		6,971			102
			Net income (loss) to common stockholders	$	62,736		$	(34,811	)
Earnings (loss) per common share:
			Basic	$	0.83		$	(0.47	)
			Diluted	$	0.76		$	(0.47	)
Shares used in calculation of earnings (loss) per share
			Basic		75,259			73,653
			Diluted		84,833			73,653

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IMCLONE SYSTEMS INCORPORATED

Consolidated Condensed Balance Sheets

(Unaudited)
(in thousands)

			March 31, 2004			December 31, 2003
Assets
Current assets:
	Cash and cash equivalents		$	194,563		$	56,850
	Securities available for sale			92,379			49,498
	Other current assets			57,027			16,222
		Total current assets		343,969			122,570
Property plant and equipment, net				265,011			245,901
Other assets				13,046			13,124
		Total assets	$	622,026		$	381,595
Liabilities and Stockholders' Deficit
Current liabilities, excluding debt			$	138,700		$	125,785
Current portion of long-term debt				240,000			—
Deferred revenue, long term				440,877			286,362
Long-term obligations				28			240,041
		Total liabilities		819,605			652,188
Stockholders' deficit				(197,579	)		(270,593	)
		Total liabilities and stockholders' deficit	$	622,026		$	381,595

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EXHIBIT 99.1

IMCLONE SYSTEMS INCORPORATED Consolidated Statements of Income (Unaudited) (in thousands, except per share data)
IMCLONE SYSTEMS INCORPORATED Consolidated Condensed Balance Sheets (Unaudited) (in thousands)