Conventional treatment of symptomatic essential thrombocythemia (ET) consists of long-term administration of myelosuppressive cytotoxic agents which, although efficacious in most cases, are associated with leukemogenic potential. Alpha-interferon (IFN) exerts a dose-dependent inhibitory influence on thrombopoiesis through a direct antiproliferative effect on megakaryocytic precursors. Therefore, it may provide a biologic, potentially non-mutagenic alternative to conventional cytotoxic treatments. At daily doses ranging from 1 to 5 M.U., alpha-IFN is efficacious in inducing a hematologic response in most patients with ET. Response to IFN is a gradual process. The median time to hematologic response varies from 1 to 3 months and a significant proportion of patients reach and maintain normal platelet counts with low doses (1-3 M.U./d). Normalization of marrow megakaryocytosis requires longer treatment (9-12 months). Also patients resistant to cytotoxic drugs may respond to alpha-IFN, suggesting a lack of cross-resistance between the two treatment modalities. Side-effects, although not severe, represents a limit to the administration of adequate doses of IFN in about 25% of cases. Once hematologic response has been obtained, both low-dose IFN and cytotoxic drugs are effective as maintenance. The full potentialities of alpha-IFN in ET in combination with cytotoxic drugs or with other cytokines need to be further investigated.