THE PATHOLOGIC and diagnostic entity "toxic megacolon" occurring in ulcerative colitis has been known for 30 years. Its evolution through description and subsequently into definition has been attributed to numerous authorities. It is not rare, but is relatively infrequent. Its incidence according to Edwards and Truelove1 was 1.6% in 624 patients reviewed by them, the focus being on complications of ulcerative colitis. In 1962, McInerney and colleagues2 reported from the Mayo Clinic 36 cases of toxic megacolon, constituting 2.9% of 1,230 patients hospitalized with ulcerative colitis during a six-year period. However, these 36 patients represented 9.5% of 379 patients hospitalized with ulcerative colitis who were ill enough to require transfusion or had a temperature of 102 F (38.9 C) or more during hospitalization. The etiologic aspects of this condition occurring in the setting of ulcerative colitis remain obscure and the mortality rate is improving only slightly. Without a