In 13 patients with severe steroid-refractory idiopathic immune thrombocytopenia (ITP), a short course of recombinant alpha 2b interferon (IFN), given at a dose of 3 MU for 12 doses, caused a significant increase in platelet count in 11 patients. The rise in platelet count occurred following completion of the short course of IFN in 10 patients and occurred during therapy in one patient. Three patients showed an increase to normal platelet counts within 14 days of discontinuing the drug, eight showed a partial response, with a platelet count increase from 30 to 100 x 10(9)/L, and two patients showed minimal response. One complete responder relapsed at 5 months from initial response, and a further course of alpha 2b IFN caused a second prompt response with a rise of platelet count to supranormal levels. Short-course alpha 2b IFN can be recommended as a therapy for severe ITP. Responses are seen in splenectomized and nonsplenectomized subjects, and thrombocytopenia is not exacerbated during treatment.