We treated 32 patients with Ph1-negative chronic myeloproliferative disorders (CMD) with excessive thrombocytosis with Interferon alpha-2b (IFN alpha-2b): 26 had essential thrombocythaemia, ET (18 previously untreated, eight pretreated); one thrombocythaemia after treatment for Hodgkin's disease (HD); two thrombocythaemia associated with non-Hodgkin's lymphoma (NHL); three stage II idiopathic myelofibrosis (IM). IFN was given at daily doses of 1-4 x 10(6) IU. Twenty-seven patients (84%) responded, 17 (53%) achieved complete haematologic response after a median time of 12 weeks, and 10 (31%) partial haematologic response. Median platelet levels declined in complete haematologic response patients from 1,190 to 335 x 10(9)/l. Normalization of megakaryocyte (MK) levels was observed in 8/17 complete haematologic response patients treated for 9-12 months, with decreased bone marrow (BM) cellularity. Side effects requiring dose reduction or discontinuation of treatment occurred in 28% of cases with IFN doses of 2 or 4 x 10(6) IU. After 1 year of continuous IFN treatment, responses were maintained with conventional chemotherapy or low-dose IFN. This study demonstrates that IFN has definite therapeutic activity in CMD with excessive thrombocytosis. This biological agent, either alone or in combination with other antineoplastic treatment, may represent a new therapeutic approach for these disorders.