Ischemic colitis has been previously described in three forms: transient, strictured, and gangrenous. A fourth form of presentation in the elderly is characterized by signs of an acute abdomen, massive colonic dilatation, and systemic toxicity. Bloody diarrhea may be seen prior to the onset of dilatation. Ischemia should be considered as an etiologic factor in “colitis” in the elderly patient with segmental dilatation particularly if it follows a “low flow state.” The rectum is usually uninvolved. Barium enema may confirm segmental involvement and later demonstrate stricture. Three patients with ischemic megacolon are presented. The diagnosis was suspected preoperatively in only one. In contrast to ulcerative colitis, these patients show a more abrupt onset and run a fulminant course. In patients who recover, there is lower relapse rate than young patients with ulcerative colitis. When resection is indicated, all attempts should be made to spare the rectum. Loop ileostomy and decompressive colostomy offer an excellent temporizing measure to assist the patient through the acute phase of the illness.