This case report documents severe autoimmune thrombocytopenia in a 20 yrs. old patient with ulcerative colitis. Diagnosis of idiopathic thrombocytopenic purpura was made on the presence of bound antiplatelet antibodies and on the absence of any other disorder capable of provoking a platelet fall. Colonic lesions were moderate but were resistant to steroids and total parenteral nutrition. Thrombocytopenia resisted to steroids and vincristine and a major fall in the platelet count (less than 10,000/mm3) occurred in two instances. However, platelets rose transiently after high-dose intravenous gamma-globulins. The patient underwent splenectomy, colectomy and ileostomy because of life-threatening colonic hemorrhage. The platelet count rose to normal levels within one week after surgery. Ileorectal anastomosis was subsequently performed. One year later the patient was in good health and his platelet count had returned to normal.