Conventional treatment of polycythemia vera (PV) with radioactive phosphorus or alkylating agents is associated with a significant excess of acute leukemia and cancer of the gastrointestinal tract and skin. There is thus a need for a nonmutagenic agent in the treatment of this disorder. Hydroxyurea (HU) was administered to 118 patients with a loading dose of 30 mg/kg/day for 1 week, which was then reduced to 15 mg/kg/day. Initial control of the elevated hematocrit and platelet count was achieved within 12 weeks in over 80% of patients. Long-term disease control was defined and the accumulative 1-year failure-free survival was 73% in the previously untreated patients and 59% in those patients previously treated with other myelosuppressive modalities. The HU was well tolerated and cytopenia, which generally occurred within the first 8 weeks of therapy, was transient and of little clinical significance. However, it is recommended because of this toxicity that HU be administered initially at a dose of 15-20 mg/kg/day. Three patients developed acute leukemia; two were untreated and one had had myelosuppressive therapy. Hydroxyurea is an effective agent in the treatment of PV, but continued assessment of its mutagenic potential is necessary.