Mortality rates during hospital stays for common diseases show considerable variation at the hospital level, which suggests that there is potential for outcome improvement. We studied changes in mortality after an intervention that aimed to improve medical outcomes through quality measurement combined with peer review. We examined eighteen acute care hospitals purchased by the Helios Hospital Group in Germany from one year before to three years after the start of the intervention. In-hospital mortality for myocardial infarction, heart failure, ischemic stroke, and pneumonia was stratified by initial hospital performance and compared to the German average. Following the intervention, hospitals whose performance was initially subpar significantly reduced in-hospital mortality for all four diseases. In hospitals that initially performed well, no significant changes in mortality were observed. The observational nonrandomized data suggest that the quality management approach was associated with improved outcomes in initially subpar hospitals. Disease-specific measures of mortality, combined with peer reviews, can be used to direct actions to areas of potential improvement.