Acute intestinal obstruction was the presenting feature in 124 (19 per cent) of 646 patients with colorectal carcinoma seen over a six-year period. Forty-two per cent of tumors were incurable at presentation. Obstruction was complicated by perforation in 22 patients (18 per cent). Only 15 per cent of tumors occurred in the rectum. Although the postoperative mortality rate was higher in patients with coincidental perforation than in those without (52 vs. 26 per cent: P = 0.03), five-year survival rates were the same: 18 per cent overall, rising to 29 to 34 per cent after "curative" resection. Five-year survival rates were best for right colon tumors and worst for rectal tumors (36 vs. 5 per cent: P = 0.01). The overall hospital mortality rates for colostomy and delayed resection, resection with colostomy, and resection with anastomosis were equivalent (18 to 22 per cent), but following "curative" resection the hospital mortality rate was higher for resection with colostomy than with other treatments (29 vs. 15 per cent), since two patients died following early closure of colostomy. Five-year survival was better following resection with anastomosis (48 per cent) than staged procedures (18 per cent: P = 0.01), since two patients died following late closure of colostomy.