A study conducted in two acute hospitals in London reviewed 1014 medical and nursing records to assess the feasibility of detecting adverse events through record review and estimate their incidence and costs. The study found that 110 (10.8%) of 1014 patients experienced an adverse event, with an overall rate of 11.7% when multiple events were included. About half of these events were judged preventable with ordinary standards of care, and a third led to moderate or greater disability or death. Adverse events resulted in 999 extra bed days, with 460 (46%) judged preventable. The total cost to the NHS of these adverse events in extra bed days alone would be around £1bn a year. The study suggests that adverse events are a serious source of harm to patients and a large drain on NHS resources. Some are major events; others are frequent, minor events that go unnoticed in routine clinical care but together have massive economic consequences. The study is primarily a pilot and has certain limitations, including being based on only two hospitals and not accurately reflecting hospital practice. However, the findings strongly suggest that adverse events are a serious problem in the NHS, as they are in the United States and Australia. The study estimates that around 5% of the 8.5 million patients admitted to hospitals in England and Wales each year experience preventable adverse events, leading to an additional three million bed days. The study concludes that a full national study would be justified in the United Kingdom, as indicated in the chief medical officer's recent report. It suggests that the investigation should cover at least 20 general hospitals and include 500 representative case records from each hospital. This would yield around 1000 adverse events for detailed analysis. Such a study would provide reliable information on the numbers, types, and costs of adverse events occurring in NHS hospitals. This would allow the principal causes to be explored and specific risk reduction strategies to be identified and costed. The total cost of such a study would probably be equivalent to the money lost through preventable adverse events in less than eight hours in the NHS.A study conducted in two acute hospitals in London reviewed 1014 medical and nursing records to assess the feasibility of detecting adverse events through record review and estimate their incidence and costs. The study found that 110 (10.8%) of 1014 patients experienced an adverse event, with an overall rate of 11.7% when multiple events were included. About half of these events were judged preventable with ordinary standards of care, and a third led to moderate or greater disability or death. Adverse events resulted in 999 extra bed days, with 460 (46%) judged preventable. The total cost to the NHS of these adverse events in extra bed days alone would be around £1bn a year. The study suggests that adverse events are a serious source of harm to patients and a large drain on NHS resources. Some are major events; others are frequent, minor events that go unnoticed in routine clinical care but together have massive economic consequences. The study is primarily a pilot and has certain limitations, including being based on only two hospitals and not accurately reflecting hospital practice. However, the findings strongly suggest that adverse events are a serious problem in the NHS, as they are in the United States and Australia. The study estimates that around 5% of the 8.5 million patients admitted to hospitals in England and Wales each year experience preventable adverse events, leading to an additional three million bed days. The study concludes that a full national study would be justified in the United Kingdom, as indicated in the chief medical officer's recent report. It suggests that the investigation should cover at least 20 general hospitals and include 500 representative case records from each hospital. This would yield around 1000 adverse events for detailed analysis. Such a study would provide reliable information on the numbers, types, and costs of adverse events occurring in NHS hospitals. This would allow the principal causes to be explored and specific risk reduction strategies to be identified and costed. The total cost of such a study would probably be equivalent to the money lost through preventable adverse events in less than eight hours in the NHS.