ObjectivesTo estimate the incidence, nature and consequences of adverse events and preventable adverse events in Swedish hospitals.DesignA three-stage structured retrospective medical record review based on the use of 18 screening criteria.SettingTwenty-eight Swedish hospitals.PopulationA representative sample (n = 1967) of the 1.2 million Swedish hospital admissions between October 2003 and September 2004.Main Outcome MeasuresProportion of admissions with adverse events, the proportion of preventable adverse events and the types and consequences of adverse events.ResultsIn total, 12.3% (n = 241) of the 1967 admissions had adverse events (95% CI, 10.8–13.7), of which 70% (n = 169) were preventable. Fifty-five percent of the preventable events led to impairment or disability, which was resolved during the admission or within 1 month from discharge, another 33% were resolved within 1 year, 9% of the preventable events led to permanent disability and 3% of the adverse events contributed to patient death. Preventable adverse events led to a mean increased length of stay of 6 days. Ten of the 18 screening criteria were sufficient to detect 90% of the preventable adverse events. When extrapolated to the 1.2 million annual admissions, the results correspond to 105 000 preventable adverse events (95% CI, 90 000–120 000) and 630 000 days of hospitalization (95% CI, 430 000–830 000).ConclusionsThis study confirms that preventable adverse events were common, and that they caused extensive human suffering and consumed a significant amount of the available hospital resources.