Long-term interferon therapy for thrombocytosis in myeloproliferative diseases

31 patients with thrombocytosis associated with myeloproliferative disorders were included in a prospective trial of long-term interferon therapy. 6 patients (19%) had side-effects which required withdrawal of interferon within one year. 22 patients (71%) achieved and maintained a complete response (platelet count less than 440 x 10(9)/l) for at least twelve months, with reduction or abolition of symptoms associated with thrombocytosis and a significant fall in bone-marrow megakaryocytes. At twelve months, 25 patients were randomly allocated to maintenance or withdrawal of interferon. Thrombocytosis recurred rapidly when treatment was stopped, but a second remission could be achieved by resumption of interferon therapy.