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This study aims to assess the extent to which acute hospital trust mergers in England are associated with quality improvements.

We apply an event study design using difference-in-difference (DID) and coarsened exact matching to compare the before-and-after performance of eight mergers from 2011 to 2015.

We find little evidence that mergers contribute to quality improvements other than some limited increases in the proportion of patients waiting a maximum of 18 weeks from referral to treatment. We postulate that financial incentives and political influence could have biased management effort towards waiting time measures.

Inherent sample size constraints may limit generalisability. Merger costs and complexity mean they are unlikely to offer an efficient strategy for helping to clear elective care backlogs. We recommend further research into causal mechanisms to help health systems maximise benefits from both mergers and emerging models of hospital provider collaboration.

This paper is the first to study the quality impact of a new wave of acute hospital mergers taking place in the English National Health Service from 2011 onwards, applying a group-time DID estimator to account for multiple treatment timings.