In 2004, the Federal Trade Commission brought legal action retrospectively challenging the 2000 acquisition of Highland Park Hospital by Evanston Northwestern Healthcare in Evanston, Illinois. A major issue in the case was whether the merger had resulted in improved clinical quality at Highland Park. In this paper, we report the findings of our analysis of that issue. We examined numerous quantitative measures of clinical quality and found little evidence that the merger improved quality. We also describe the conceptual framework in which we evaluated the case-specific evidence, as well as the applicability of that framework to the prospective analysis of unconsummated hospital mergers.