Fifty consecutive patients, 30 of whom had polycythaemia vera (PV), 10 essential thrombocythaemia (ET), and 10 myelofibrosis (MF), entered a long-term prospective study of hydroxyurea (HU) therapy. The indication for treatment was mainly thrombocytosis or symptomatic splenomegaly. Control of erythrocytosis and thrombocytosis was achieved in 70% of the patients. Continuous maintenance treatment was required. In 15% of responding patients with thrombocytosis, unexpected rises of the platelet count occurred during maintenance therapy. Severe thrombo-embolic events occurred in 6 patients. The size of the spleen decreased in all patients who did not develop thrombocytopenia and could absorb adequate HU doses. Acute leukaemia (AL) was diagnosed in 9 patients and a myelodysplastic syndrome in one. Seven of them had been treated with HU alone. Among the patients with PV and ET, 6 developed AL and 4 of them were treated with HU alone (3 PV and 1 ET), giving an incidence of 10.5%. In previously untreated patients with initially normal karyotypes (n = 19), chromosome abnormalities developed during HU therapy in 7 (37%). Our results indicate that HU should be regarded as leukaemogenic, at least when used for treatment of PV and allied diseases. Since myelosuppressive treatment of PV does not prolong survival, the use of HU should be restricted to patients in whom the treatment indication outweights the risk of leukaemia induction.