We examine the determinants and effects of M&A activity in the pharmaceutical/ biotechnology industry using SDC data on 383 firms from 1988 to 2001. For large firms, mergers are a response to expected excess capacity due to patent expirations and gaps in a firm’s product pipeline. For small firms, mergers are primarily an exit strategy in response to financial trouble (low Tobin’s q; few marketed products, low cash–sales ratios). In estimating effects of mergers, we use a propensity score to control for selection based on observed characteristics. Controlling for merger propensity, large firms that merged experienced a similar change in enterprise value, sales, employees, and R&D, and had slower growth in operating profit, compared with similar firms that did not merge. Thus mergers may be a response to trouble, but they are not a solution.