We studied a patient with thrombocytopenia associated with Hodgkin's disease (HD). The megakaryocyte number in the bone marrow and the level of platelet-associated IgG were both increased in the patient. Intravenous gamma-globulin therapy and chemotherapy for HD dramatically normalized the platelet count, suggesting that antibody produced by lymphoma cells is likely to account for the thrombocytopenia. Antigen-captured ELISA and Western blotting showed that the patient's serum had an IgG autoantibody against platelet membrane glycoprotein Ib. The patient's plasma had no inhibitory effect on normal platelet aggregation induced by ristocetin. These findings suggest that the autoantibody found in the patient had a pathogenetic role in the thrombocytopenia, but not in platelet dysfunction.