During an 11-year period from January 1970 to December 1980, 7,199 patients at our institution had chronic ulcerative colitis or Crohn's disease. Thromboembolic complications developed in 92 (1.3%) of these patients. An additional 4 patients had cutaneous vasculitis, and 17 had an arteritis-associated diagnosis. Of the thromboembolic complications, 61 were deep vein thromboses or pulmonary emboli. The mortality among patients with thromboembolic complications was high (25%). Sixty percent of the patients had a thrombocytosis unaffected by sulfasalazine or corticosteroid therapy. In 73% of the patients, the erythrocyte sedimentation rate was increased, and when measured, fibrinogen and factor VIII were commonly elevated. Peripheral arterial thrombosis, coronary thrombosis, and mesenteric and portal vein thrombosis were predominantly postsurgical complications, but 77% of peripheral venous thromboses occurred spontaneously. The role of anticoagulation and surgical intervention in the management of hypercoagulation in patients with inflammatory bowel disease is discussed.