Wound infection rates in a general hospital over an 11-year period have been studied, the review embracing over 12,000 sutured surgical wounds. There is a remarkably constant downward trend in the wound infection rate. This is considered real, as there has been no change in the method of recording. Changes in operating theatre technique have not individually influenced the rates or the trend. In particular, the use or non-use of surgical masks made no difference to the incidence of wound infection. Changes in scrub-up-technique and the introduction of air-conditioning in the operating rooms had no appreciable effect. There is a difference between wound infection rates of acute operations and elective operations, of "clean" operations and "dirty" operations. The possible sources of surgical wound infection are discussed. The classification into "inherent" and "sporadic" groups is offered, and in "sporadic" infections the role of the patient as a source of infection is considered.