We examine the effects of mergers on the returns to acquiring companies' shareholders for a large sample of companies from both Anglo-Saxon and non-Anglo-Saxon countries over the 1980s and 1990s. With the important exception of Japan, we find similar patterns of returns across both types of countries. For a sample of 9733 acquiring companies the mean percentage gain over a short window of 21 days is 0.6%. This picture changes dramatically as the market has more time to evaluate the mergers and/or the acquiring firms. After three years, acquirers' shareholders in the United States and continental Europe lost on average 19% of their market value compared to a portfolio of non-merging firms in their size deciles and their two-digit industry, in Canada, Australia and New Zealand roughly 16%, and in the four Scandinavian countries almost 15%. Further analysis indicates that some mergers are consistent with the hypothesis that mergers generate synergies, but that a majority of mergers in Continental Europe are explained by the managerial discretion and/or hubris hypothesis. Our findings also suggest that corporate governance institutions in the United States and the other Anglo-Saxon countries lead to better investment performance than in continental Europe, when one confines one's attention to mergers. Copyright © 2007 John Wiley & Sons, Ltd.