Twelve patients with hairy cell leukemia underwent treatment with beta-ser interferon, a type I interferon with potent antiproliferative activity. Ten of the 12 patients had had prior therapy and the median time from diagnosis to initiation of treatment was 8.5 months (range 1 month to 18 years). Bone marrow involvement ranged from 90-100% hairy cells. Beta-ser interferon was administered as an intravenous injection of 90 million units per dose, three times weekly. Dose reductions were by dose and by frequency in individual patients. Ten of 12 patients (83%) responded, including 7 complete (CR) and 3 partial (PR) responses. The median time to at least PR was 6 months and the median time to CR was 12 months. The median duration of overall response (PR + CR or PR) is 31+ months (range 26(+)-40+ months). Of the 6 patients who underwent dose reductions due to toxicity or poor tolerance, 4 CR patients reverted to PR, and 2 PR patients have increased the number of hairy cells in the marrow, while maintaining normal peripheral blood counts. One each of these has regained CR and PR status after an increase in dose.