Ulcerative colitis (UC) is associated with extraintestinal diseases in numerous target tissues. Associated immune-mediated hematological diseases, however, are rarely described. We report three Caucasian adult patients with UC and immune thrombocytopenic purpura (ITP). Platelet-associated antibodies (IgG) were positive in two patients, and bone marrow examinations in two patients revealed normal to increased megakaryocyte numbers. ITP was treated with corticosteroids in all patients. Two patients eventually received intravenous immune gamma-globulin, and one patient required surgical splenectomy. Of particular interest, UC preceded the onset of ITP in all patients (by from 1 to 19 yr). This suggests that ITP in these patients is causally associated with UC, possibly secondary to immunostimulation from lumenal antigens and altered immunoregulation.