On the basis of a study of the clinical records of 1,312 patients operated on for possible appendicitis over a 2 year period, morbidity of a potentially serious nature was found to involve between 10 per cent (patients having normal appendices removed) and 75 per cent (patients having perforated appendices removed) of all patients operated on. These values include an estimate of the risk of requiring a second operation (mainly for adhesion obstruction) before and beyond 2 years after the original operation; in each instance this was calculated to be approximately 1 per cent. Advanced appendicitis is commoner in older than younger patients; in young patients more males than females have advanced appendicitis and more females than males have limited acute appendicitis. The overall incidence of appendicitis in young males and females appears to be equal. Removal of a normal appendix carries a significantly better prognosis if the operation is an emergency procedure (86% cured) as compared with a planned procedure (71% cured).