Interferons are not curative in hairy cell leukemia (HCL), and retreatment is necessary in most patients whose therapy is stopped. In an attempt to maintain or improve responses, we administered recombinant interferon-alpha 2a (rIFN-alpha 2a) continuously to patients with HCL who initially responded to this therapy. Of 53 evaluable patients enrolled in this study, 32 have received rIFN-alpha 2a continuously for a median of 5 years. Patients received 3 million units of rIFN-alpha 2a subcutaneously (SC) daily for 6 months, followed, in responding patients, by the same dose three times weekly. Twenty-one patients (40%) discontinued IFN after a median of 29 months, seven of whom developed resistant disease in association with anti-IFN antibodies. Treatment produced high response rates: complete response plus partial response (CR + PR) = 40 of 53 (76%), CR + PR + minor response (MR) = 43 of 53 (82%), with no differences in response rates between patients with and without splenectomy. Sixteen patients who had MR at 18 months had PR with prolonged treatment, nine of whom had a significant further reduction in the hairy cell infiltrate in the bone marrow (BM). The median granulocyte and platelet counts have continued to increase and the median serum soluble interleukin-2 receptor (sIL-2R) level has continued to decrease with prolonged treatment. Two patients developed erythrocytosis that may be treatment related, but no other new toxicities were noted with prolonged treatment. We conclude that prolonged, continuous rIFN-alpha 2a treatment has acceptable toxicity, is not associated with late development of IFN resistance, and results in continued hematologic improvement with time on treatment.