Objective. The prevalence and variability of adverse outcome rates in Belgian acute hospitals is examined by using the national hospital discharge database. Design, setting, and participants. Retrospective analysis based on administrative data of all Belgian acute hospitals, covering the full medical ( n = 1 024 743) and surgical ( n = 633 027) in-patients population for the year 2000. Main outcome measures. For 11 adverse outcomes and failure-to-rescue, the rates and variability among hospitals were studied. The all patient refined diagnostic-related groups (APR-DRG) method was used for risk adjustment. Results. The prevalence of adverse outcomes was 7.12% in the medical and 6.32% in the surgical group. Rates ranged from 6.25 (deep venous thrombosis) to 32.3 (urinary tract infection) outcomes per 1000 discharges in the medical group and from 3.39 (deep venous thrombosis) to 17.6 (urinary tract infection) outcomes per 1000 discharges in the surgical group. The failure-to-rescue rate was 240 and 211 per 1000 discharges, respectively. Except for pressure ulcers and hospital-acquired sepsis, the prevalence of adverse outcomes was significantly higher ( P = 0.001) in the medical group. All adverse outcome rates varied substantially among the hospitals surveyed. Conclusions. This study identifies the occurrence of adverse outcomes in a national population. It adds information to the growing body of knowledge in predominantly Anglo-Saxon countries about adverse outcomes. Striking variation exists in the risk-adjusted adverse outcome rates across Belgian acute hospitals, revealing a large potential for quality gains that encourage further action.