A prospective study was made of 23,649 surgical wounds. All wounds were examined by one person for 28 days after operation. National Research Council definitions and classifications were used. The overall infection rate was 4.75%, the clean rate was 1.81%. Sepsis became evident only after the patient had left the hospital in 13.2% of cases. Analysis of 15,389 clean wounds showed a reduction in the rate of infection in those using a hexachlorophene wash before operation and those whose operation site was not shaved. No reduction or increase in the rate of infection was observed when plastic skin drapes were used or when different hand-scrub preparations were used. Increase in the rate of infection occurred in old age, when drains were used, when there was a prolonged hospital stay before the operation, and when the duration of the operation was great.