Acute necrotizing colitis is a rare complication of colonic obstruction. Six cases occurring during a 20-month period are described. The presenting features were those of colonic obstruction with shock but without perforation. At laparotomy changes ranged from mucosal necrosis to frank gangrene in the colon proximal to the obstructing lesion. Gram stains of resected colon showed Gram-positive bacilli, resembling clostridia, invading the mucosa and submucosa. Two patients treated by defunctioning colostomy alone died but the remaining 4 survived after total colectomy. The cause is not known but raised intraluminal pressure may result in terminal mucosal ischaemia allowing anaerobic organisms to invade the bowel wall.