Eighteen patients with polycythemia vera who were less than 60 years old received human leukocyte interferon-α subcutaneously at a starting dose of 3 MU three times a week. The interferon dose was escalated to 6 MU three times a week if it was well tolerated and disease was not controlled after 3 months of treatment at the lower dose. Hematologic response was defined as complete if the hematocrit was maintained at less than 45% in the absence of phlebotomy and partial if the hematocrit was kept at 45% to 50%, associated with a 50% or greater reduction of phlebotomy requirements; no response was defined as a response less than a partial response. Complete disease control was achieved in 11 patients, with partial control in a further six cases. One patient failed to respond. Median duration of response was 16 months (range 5 to 43 months), with 15 patients still under treatment. Therapy with human leukocyte interferon-α significantly improved (p<.01) phlebotomy requirements, the degree of splenomegaly, pruritus scores, iron stores and mean red cell volume values, and platelet and leukocyte counts. Interferon treatment did not produce remarkable side effects and no patient withdrew from the study because of intolerance. We conclude that subcutaneous human leukocyte interferon-α is an effective and well-tolerated therapy in the management of polycythemia vera-associated myeloproliferation and pruritus in patients less than 60 years old.