An analysis of surgical wound infections according to type of operation and bacteria involved, based on a prospective study of 20,822 operations folluwed at the t'rincess Alexandra Hospital from 1963 to 1971, is reported. 1 he overall infection rate for clean general surgery (Class “A” operations) was 4·5%; for potentially infected surgery (Class “B” operations) 9·5%; and for clean orthopcedtc surgery 3%. 1he infection rates variea from month to month and year to year without any consistent pattern. Staphylococcus aureus was the commonest infecting organism in Class “A” operations and coliform bacilli in Class “B”. Care must be taken before assuming that a drop in the incidence of wound infection is due to the introduction of any particular measure, such as a change in technique or the administration of an antibiotic.