Net Loss Per Common Share (Tables)

3 Months Ended

Mar. 31, 2013

Net Loss Per Common Share

Schedule of computation of basic and diluted net loss per share available to common stockholders

	Three months ended March 31,
(in thousands, except per share amount)	2012			2013
Net Loss Per Share:
Numerator:
Net loss attributable to Merrimack Pharmaceuticals, Inc.	$	(23,284	)	$	(28,153	)
Plus: Unaccreted dividends on convertible preferred stock	(2,040		)	—
Net loss available to common stockholders—basic and diluted	(25,324		)	(28,153		)
Denominator:
Weighted-average common shares—basic and diluted	11,846			95,879
Net loss per share available to common stockholders—basic and diluted	$	(2.14	)	$	(0.29	)

Schedule of potentially dilutive securities excluded from computation of diluted weighted average shares

	As of March 31,
(in thousands)	2012	2013
Convertible preferred stock	66,256	—
Options to purchase common stock	17,515	20,664
Convertible preferred stock warrants	302	—
Common stock warrants	2,631	2,779