Immune thrombocytopenia occurred in five patients with cancer receiving α-interferon. The median pretreatment platelet count was 217,000/cu mm, which fell to a median of 12,000/cu mm after a median of 25 days of interferon therapy. All had normal numbers of megakaryocytes, with dysplasia noted in three; all four who were tested had platelet-associated immunoglobulin. All had normalization of platelet counts with prednisone therapy. Four tolerated re-treatment with interferon, two with concurrent prednisone administration and two others following splenectomy. Immune thrombocytopenia should be considered in patients who become thrombocytopenic during interferon therapy.