Interferon alpha (INF alpha) is the drug of choice for patients where the interval between the diagnosis and INF alpha administration is not longer than one year. The usual dosage is 5 x 10(6) i.u./m2 INF-alpha s.c. per day. Haematological and cytogenic criteria are applied to evaluate treatment. INF alpha prolongs the life of patients with a cytogenic response. In these patients treatment should be continued till signs of relapse develop. In patients without a cytogenic response INF alpha does not substantially prolong the survival period. INF alpha is indicated in all myeloproliferative diseases associated with thrombocytosis. It holds its place in the treatment of patients with primary myelofibrosis. In both these indications it is recommended to start INF alpha treatment-3 x 10(6) i.u. INF alpha per day. In polycythemia vera INF alpha reduces the number of erythrocytes. It is not clear, however, whether it prolongs life.