To determine whether hospital ownership was associated with preventable adverse events, the authors reviewed the medical records of a random sample of 15,000 hospitalizations in Utah and Colorado in 1992. Hospitals were categorized as nonprofit, for-profit, major teaching government (e.g., county, state ownership), and minor or nonteaching government. Multivariate analyses adjusting for other patient and hospital characteristics found that, when compared with patients in nonprofit hospitals, patients in minor or nonteaching government hospitals were more likely to suffer a preventable adverse event of any type (odds ratio (OR), 2.46; 95 percent confidence interval (95% CI), 1.45 to 4.20); preventable operative adverse events (OR, 4.85; 95% CI, 2.44 to 9.62); and preventable adverse events due to delayed diagnoses and therapies (OR, 4.27; 95% CI, 1.48 to 12.31). Patients in for-profit hospitals were also more likely to suffer preventable adverse events of any type (OR, 1.57; 95% CI, 1.03 to 2.38); preventable operative adverse events (OR, 2.63; 95% CI, 1.42 to 4.87); and preventable adverse events due to delayed diagnoses and therapies (OR, 4.15; 95% CI, 1.84 to 9.34). Patients in major teaching government hospitals were less likely to suffer preventable adverse drug events (OR, 0.38; 95% CI, 0.16 to 0.89).