A 35-year-old pregnant woman had thrombocytopenia with a platelet count of 6.3 x 10(4)/microliter. After her third normal delivery, peripheral blood studies revealed that the patient had a normal Hb concentration and leukocyte count, with mild thrombocytopenia. A diagnosis of idiopathic thrombocytopenic purpura (ITP) was made based on the high megakaryocyte count of 338/microliter and PAIgG of 40.8 ng/10(7) cells in January 1995. The patient was followed without treatment. She was 9 weeks pregnant on June 7, 1996, and desired an abortion. Her platelet count was 6.3 x 10(4)/microliter, leukocyte count 8,600/microliter, and Hb 13.7 g/dl at the time. She was given high-dose intravenous gammaglobulin (Globenin-I) at 400 mg/kg/day for 5 consecutive days. The platelet count was found to have decreased markedly, to 0.9 x 10(4)/microliter on June 11. The percentage reduction in the Hb concentration, leukocyte count, and platelet count after gammaglobulin treatment was 11.7%, 46.6%, and 85.8%, respectively. The PAIgG titer had increased to 181.2 ng/10(7) cells on June 17, but hypergammaglobulinemia was suspected. The patient was started on prednisolone on June 24, and an abortion was performed on July 29. The mechanism of thrombocytopenia after infusion of Globenin-I was unknown. We suspect that Globenin-I treated with polyethylene glycol was one of the possible causes of myelosuppression in this case.