In this paper we are trying to study the influence of mergers and acquisitions in the returns of the involved companies. Our research use data from Athens Stock Exchange market in order to analyze and to interpret the differences between the actual and expected returns. Our results have shown that the stock risk of Alpha Bank in the case of acquisition by Ionian Bank is aggressive. The same results we have and in the case of acquisition Alpha Bank with the National Bank of Greece. This means that the changes of stock are more intense than the changes of the market. On the contrary, for the cases of National Bank of Greece with the National Ktimatiki Bank of Greece, Piraeus Bank with the Macedonia – Thrace Bank and the Eurobank with the Ergasias Bank, the stock risk is defensive, which means that the changes of stocks of the National Bank of Greece, Piraeus Bank and Eurobank are smaller than the changes of market (defensive stocks).

KEYWORDS: - Mergers, Acquisitions, Greece, Risk, Banks.