The study examines the likelihood of adverse outcomes associated with selected hospital safety events for two groups of Medicare patients: those enrolled in health maintenance organizations (HMOs) versus those enrolled in fee-for-service (FFS) insurance plans. The authors hypothesize that HMO patients may receive different qualities of hospital services and/or physician services relative to FFS patients. Based on the Healthcare Cost and Utilization Project State Inpatient Database, the authors include discharge data on all hospitalized elderly Medicare patients in Florida in 2002 and use multivariate logistic regression models with adjustments for hospital-level clusters. The findings demonstrate that, after adjusting for hospital quality, Medicare HMO patients were at higher risk of adverse outcomes than Medicare FFS patients for iatrogenic pneumothorax, accidental puncture or laceration, and postoperative respiratory failure.