We report on long-term therapeutic efficacy and toxicity of recombinant interferon-alpha 2a (rIFN-alpha) in a series of 38 patients with polycythaemia vera (PV). In all patients haematocrit was first brought into the normal range by venesection; rIFN-alpha was then begun at a starting weekly dose of 9,000,000 IU. Complete response (CR) was defined as persistence of normal haematocrit without venesection and partial response (PR) as >50% reduction of phlebotomy requirement. Eleven patients (28.9%) achieved CR and 8 (21.0%) PR. Median duration of treatment for all responsive patients was 40 months; 12 patients are still responsive and under treatment after 13, 15, 25, 35, 40, 41, 43, 49, 50, 51, 52 and 52 months of therapy with rIFN-alpha. In responsive patients, rIFN-alpha also normalized leucocyte counts, platelet counts and spleen enlargement; rIFN-alpha also relieved generalized pruritus in all 10 patients displaying this symptom. Early toxicity (flu-like syndrome) was observed in 23.6% and late toxicity (severe weakness) in 13.1% of patients, requiring rIFN-alpha treatment suspension in all cases. Progression to leukaemia was observed in none of the 10 patients treated only with rIFN-alpha and in one of the 12 who received alkylating agents before enrolment in this study. According to these data, rIFN-alpha seems to be an effective and safe treatment option for PV.