High-dose gammaglobulin therapy for patients with idiopathic thrombocytopenic purpura (ITP), introduced by Imbach et al., was applied to 5 adults with chronic refractory ITP to investigate the mechanism of the increase in platelet counts. In 4 of the 5 cases, transient increase in platelet count was observed. Platelet-associated IgG was decreased in 3 cases, increased in 1 case and unchanged in 1 case after treatment. In 4 cases having a variety of autoantibodies, the antibody titers decreased after treatment. No significant changes in antiplatelet antibody titers in serum, or in circulating immune complexes were observed during or after these treatments. No side effects were noted in any of the cases. These results indicate that the treatment is suitable for the treatment of patients prior to surgery and of patients at high risk of intense bleeding and death. It can be reasonably assumed that the increased platelet count is mainly due to a defective removal of antibody-coated platelets.