Free perforation of the colon and rectum is an acute surgical condition. Possible factors affecting the prognosis include age, sex, underlying disease, surgical procedures, duration from onset to surgical treatment, general condition before surgery and degree of contamination. A series of 50 cases with acute, nontraumatic perforation of the colon and rectum treated at Chang Gung Memorial Hospital from 1979 to 1986 were reviewed and studied retrospectively according to these prognostic factors. The mortality rate was highest in the group with colo-rectal cancer (45%). The mortality rate was 50% in the group who underwent primary closure with proximal diversion and 40% in the group who underwent resection without anastomosis. The mortality may not be related to the surgical procedure: the selection of the procedure was based on the seriousness of the illness. The mortality rate was 87% in patients with septic shock, 62% when treatment was delayed for more than 72 hours, 72% with severe contamination and 56% with poor nutritional status. Age, sex and underlying diseases were not significant contributing factors.