This systematic review examines what factors explain the diversity of findings regarding hospital ownership and quality. We identified 31 observational studies written in English since 1990 that used multivariate analysis to examine quality of care at nonfederal general acute, short-stay US hospitals. We find that pooled estimates of ownership effects are sensitive to the subset of studies included and the extent of overlap among hospitals analyzed in the underlying studies. Ownership does appear to be systematically related to differences in quality among hospitals in several contexts. Whether studies find for-profit and government-controlled hospitals to have higher mortality rates or rates of adverse events than their nonprofit counterparts depends on data sources, time period, and region covered. Policymakers should be aware of the underlying reasons for conflicting evidence in this literature, and the strengths and weaknesses of meta-analytic synthesis. The "true" effect of ownership appears to depend on institutional context, including differences across regions, markets, and over time.