Thromboembolism as a complication of ulcerative colitis has received scant attention in the medical literature. In most treatises on the disease either it is not mentioned at all or it receives only passing notice. Between 7% and 30% of patients with ulcerative colitis are said to develop nonpostoperative intravascular thromboses. Usually only the more seriously ill patients are affected. That phlebitis and arterial occlusion may be a serious and, at times, grave occurrence in ulcerative colitis is borne out by the following two cases in both of which autoimmune mechanisms may have played an important role. Report of Cases Case 1. —This 20-year-old white soldier with two years of active military service was in good health until March, 1961, when he noted fatigue, loss of energy, pallor, and increased frequency of bowel movements. Initially, he passed two to six loose stools daily which, at times, contained bright red blood. There