This article explores merger integration processes in the Polish banking sector and the way in which the merged banks cope with difficulties emanating from firm-specific and nation-specific differences. The first part of the article analyzes mergers and acquisitions that have taken place in Poland from 1997 and attempts to measure the reaction of the capital market to the merger announcements, estimate changes in profitability and cost ratios, and investigate the development of productivity. Three factors are identified that define the significance of merger integration process: (1) the degree of compatibility of administrative practices, organizational structures and organizational cultures; (2) the kind and degree of post-merger consolidation; (3) the nature of the relationship between the two organizations. The results indicate that merged banks in Poland have increased profitability and shareholders have also experienced positive abnormal returns. However, the improvements in cost-efficiency were not so clear. Acquisitions were less successful than mergers in transferring banking practices and were probably motivated by the quest for additional market power.