Sixty-four patients with hairy-cell leukemia (HCL) (61 had undergone prior splenectomy) were treated with alpha-2 interferon (Intron A, Schering Corp, Kenilworth, NJ) subcutaneously three times per week at a dosage of 2 X 10(6) U/m2. Three patients (5%) demonstrated a complete response (CR) with apparent eradication of hairy cells from the bone marrow, 45 patients (70%) showed a partial response (PR) defined as normalization of all three blood counts associated with decreased involvement in the bone marrow, and nine patients (14%) showed a minor response that included improvement in at least one blood count. Three patients had no response, three patients died before completing 1 month of therapy, and one patient refused further therapy after 1 month of therapy. The median platelet count returned to normal by the second month of treatment. The median hemoglobin returned to greater than 12 mg/dL by the fourth month of treatment, and the median granulocyte count to greater than 1,500/mu by the fifth month of treatment. Bone marrow biopsy analysis during interferon therapy demonstrated a decrease in median hairy-cell index by more than half. Transfusion of both RBCs and platelets were decreased within 4 months of initiating treatment. Serious infections, which averaged four per month in 16 of the 64 patients before interferon therapy, were rarely observed after the first month of treatment. Treatment-induced toxicity was mild, consisting primarily of influenza-like symptoms, fatigue, and minor skin disorders. Alpha-2 interferon therapy is highly effective in reversing the course of progressive HCL and should be considered the treatment of choice for a minimum of 12 months in patients who have progressive disease post-splenectomy.