Despite the current wave of US hospital mergers, it is unclear how they change behaviour and performance. This column ‘opens the black box’ of hospital practices by analysing a mega-merger between two for-profit chains. Benchmarking the merger’s actual effects against the acquirer’s stated aims, its shows that while the merger achieved some goals – harmonising electronic medical records and sending managers to target hospitals – it produced few gains in overall performance. Further research is needed into the impact of mergers on the healthcare industry specifically and the economy more broadly.