This work aims to verify the return that an M and A event brings in the Brazilian capital market to the shareholder in the medium and long term. The purpose of this study is to confirm, through an empirical study, if the shareholder of a company has or not your wealth increased after a merger or acquisition. First, a theoretical framework on the subject of M and A is presented. The basic assumption of this paper is to validate or not the theory of maximizing shareholder wealth, which is the basis for the theory of mergers and acquisitions (M and As). The maximization of shareholder profits theory says that a merger or acquisition should increase shareholder wealth. In Brazil there are few studies which analyze what happens to the return for shareholders after an M and A event in the medium and long term. The methods used for this study were the study of events and the regression difference in differences. Daily closing prices were used to observe the abnormal returns in periods of up to 50 days before and after the event announcement - for the event study. However for the diff-in-diff regression window expands from event announcement to October/2015. The purpose of both approaches is to check if the shareholder had or no increase in their wealth due to the event. At the end of the study it is concluded that shareholders of Brazilian capital market, on average, increased their wealth, and this increase is related to the event of merger or acquisition.