A prospective randomized trial was carried out on 62 patients with diffuse peritonitis from perforated diverticulitis of the left colon, comparing acute transverse colostomy, suture and omental covering of a visible perforation with acute resection without primary anastomosis. For purulent peritonitis the postoperative mortality rate was significantly higher after acute resection (six of 25) than after colostomy (none of 21). In those treated by acute resection, the mortality rate was not significantly higher after Hartmann's procedure (five of 15) than after exteriorization of both lumens (one of ten). The postoperative mortality rate in patients with faecal peritonitis did not differ significantly between colostomy (six of ten) and acute resection (two of six). Stomas became permanent in four of 25 patients with diverticulitis surviving acute colostomy and in seven of 22 surviving acute resection. Suture and transverse colostomy is superior to resection for purulent peritonitis because of the lower postoperative mortality rate and in spite of the shorter hospital stay in those surviving acute resection.