A prospective four-year study on the infection rate of clean operative wounds is presented. From January 1982 to June 1985, a nurse epidemiologist and a medical team assessed 4,468 operative procedures, from the day of surgery to the patients' discharge from the hospital. The infection rate was 3.2%. A higher incidence of wound infection was detected in patients requiring emergency operations (5.1%), in drained wounds (5.4%), and in patients with conditions thought to predispose to infection, such as advanced cancer, hepatic cirrhosis, diabetes, nephrotic syndrome, previous splenectomy, and treatment with immunosuppressive drugs (7.8%). Age over 65 did not influence infection rates. There were up to tenfold differences in infection indices between surgeons performing the same clean procedures. The continued monitoring of clean wound infection rates allowed the early detection and control of infection outbreaks. Providing periodic information on infection rates to the different surgical services was associated with decreasing infection rates over time.