During the course of our time, most diseases have begun to enjoy the dignity of their own etiology and classification. In contrast, ulcerative colitis and related diseases such as ileocolitis, regional enteritis, and Crohn's disease remain in a state of confusion. Perhaps the lack of unanimity of opinion as to terminology, classification, and therapy is due to the fact that the etiology of these ulcerative diseases remains unknown. It is not strange then that the several complications which occur within the category of ulcerative diseases of the bowel are likewise poorly understood. Unique among these complications is the acute dilatation of the colon often referred to as toxic megacolon. We have chosen to analyze this complication with the desire to correct certain misconceptions. Such an analysis, therefore, should deal with the evolution of the dilatation of the ulcerative colon, the impact of the complication on the patient, and a rational