There is diverging empirical evidence on the competitive effects of horizontal mergers: consumer prices (and thus presumably competitors' profits) often rise while competitors' share prices fall. Our model of endogenous mergers provides a possible reconciliation. It is demonstrated that anticompetitive mergers may reduce competitors' share prices, if the merger announcement informs the market that the competitors' lost a race to buy the target. Also the use of first rumor as an event may create similar problems of interpretation. We also indicate how the event-study methodology may be adapted to identify competitive effects and thus, the welfare consequences for consumers.