Purpose: Merger approving focuses on both market power and welfare gains. In general, the approval process does not include a comparative efficiency analysis. This paper aims to introduce this dimension and show its potential. Design/methodology/approach: Based on the analysis of past bank mergers, the authors examine expected and actual efficiency gains. This paper measures the potential (ex ante) and ex post efficiency gains of bank mergers by using data envelopment analysis (DEA). Findings: The authors find some (approved) mergers were promised and yielded efficiency gains while others did not. Research limitations/implications: DEA does not allow testing statistically the significance of the presumed relationship between variables. Prac...