Abstract We conducted a one-year prospective survey to identify adverse outcomes due to error during care in the field of general surgery. We identified 36 such cases among 5612 surgical admissions to the Peter Bent Brigham Hospital, but in 23 cases the initiating mishap had occurred in another hospital before transfer. In two thirds of the cases the mishap was due to an error of commission: an unnecessary, defective, or inappropriate operative procedure. Twenty of these patients died in the hospital, and in 11 death was directly attributable to the error. Five of the 16 survivors left the hospital with serious physical impairment. A satisfactory outcome was achieved in only 11 cases (31 per cent). The average hospital stay was 42 days, with the duration ranging from one to 325 days; the total cost for the 36 patients was $1,732,432. We suggest that all hospitals develop comprehensive methods to identify and prevent these costly and unnecessary events. (N Engl J Med. 1981; 304:634–7.)