Extraintestinal manifestations of ulcerative colitis (UC) are well known, but immunologically mediated hematological diseases are relatively rare. We describe two cases of immune thrombocytopenic purpura (ITP) associated with preexisting UC. Our patients had typical symptoms of UC, and endoscopy showed pancolitis. During treatment with 5-aminosalicylic acid and steroids, severe thrombocytopenia was noted. ITP was diagnosed based on a normal to high number of megakaryocytes in the bone marrow, positive autoantibody to platelet membrane antigen, and absence of splenomegaly. Medical treatment, including increased dosage of steroids, failed to control UC and ITP in both patients. In the first patient, the platelet count recovered after colectomy, while the second patient died of a cerebral hemorrhage. We stress that a diagnosis of ITP should be considered for thrombocytopenia in patients with UC, especially those showing extensive and significant colonic inflammation, and that colectomy of UC might resolve resistant ITP.