In order to analyze the usefulness of different types of treatment in relation to the interval since the onset of idiopathic thrombocytopenic purpura (ITP), a collaborative study of 934 adult patients was undertaken. Prednisone was administered to 818 patients, and 32% of them achieved prolonged complete remission (PCR). However, only 14% of patients who had ITP for more than six months achieved a prednisone- induced PCR (P less than .01). Splenectomy was done in 399 patients, and 65% of them achieved PCR; the remission rate did not vary with the interval since the onset of ITP. Of 120 patients with chronic ITP that was refractory to corticosteroids and splenectomy, 91 received either azathioprine or cyclophosphamide; 21% of them achieved PCR and 55% had a favorable response. None of 19 patients treated with vincristine and only one of ten patients treated with vinblastine-loaded platelets achieved PCR.