A multicenter prospective study on the treatment of chronic idiopathic thrombocytopenic purpura (ITP) was conducted by the Idiopathic Disorders of Hematopoietic Organs Research Committee (IDHORC), the Ministry of Health and Welfare of Japan. The aim of the study was to establish an improved therapeutic guide for chronic ITP. Of the 247 eligible patients 175 have been followed up to the present time, 16 patients have died, and 56 have been lost to follow-up. The median follow-up time was 55 months (range: 1 to 91 months). Of the 206 patients treated with corticosteroids, 13.1% achieved sustained complete remission. Splenectomy produced remissions in 52.5% of 72 patients evaluated 12 months after operation. Eighty-five patients were treated with immunosuppressive agents, but the response rates were low and the effect was transient in most cases. In the patients followed up without any specific treatment, the mean platelet counts showed slight improvement. Of the 16 patients who died during the observation period, only one died of bleeding; the remaining 15 died of causes unrelated to thrombocytopenia. It is noteworthy that five patients who were treated with corticosteroids or immunosuppressive agents died of infection.