Interferon-alpha (IFN-alpha) exhibits a clear platelet reductive effect in patients with essential thrombocythemia as well as in other chronic myeloproliferative disorders with thrombocytosis. In a total of 51 patients with chronic myeloproliferative disorders with thrombocytosis we analyzed the effect of IFN-alpha in respect to platelet reduction, remission rates, induction- and maintenance dosage, long term tolerance and side effects. According to our classification CML 6, chronic mega-karyocytic granulocytic myelosis 5, essential thrombocythemia 26 and polycythemia vera 15 patients were treated. Treatment consisted of induction with 3 or 5 MU IFN-alpha daily followed by a maintenance therapy with 3 or 5 MU thrice weekly. Platelet reduction was found in all patients, CR (platelets < 450 G/l) in 78%. Within 2 months of induction therapy, CR in patients treated with 5 MU IFN daily was found in 75% compared to 52% in patients treated with 3 MU IFN daily. Dosage reduction in maintenance periode caused an increase of platelets to more than 450 G/l in 39% of patients. Out of 40 Philadelphia-negative chronic myeloproliferative disorders treated for more than 3 months in 10 patients treatment was disrupted after 5 to 18 months because of the following side effects: nausea, fatigue, vertigo, fever, headache, diarrhea, anorexia, heartburn, hairloss, myalgia, and thrombocytopenia. Due to the mutagenic effect of alkylating cytostatics and Radiophosphorus, IFN-alpha treatment represents a first line strategy for chronic myeloproliferative disorders with thrombocytosis especially in younger patients who are symptomatic and in those who suffered from episodes of bleeding or thrombosis.