Eighty-five hairy cell leukemia (HCL) patients who had failed initial therapy with recombinant alpha interferon were enrolled for pentostatin therapy. All had HCL confirmed by central pathology review and were eligible for evaluation. The median age was 55 years (range 31-83 years). Fifteen patients were between 31- and 40-years-old. There were 72 males (85%) and 13 females (15%). Fifty-four patients (64%) had prior splenectomy. All had been previously treated with interferon; nine had achieved a complete response (CR), 36 had a partial response (PR), 35 had stable disease (SD), and five patients had progressive disease. Patients with a CALGB performance status (PS) of 0-2 (78 patients) received 4 mg/m2 i.v. on days 1 and 15, repeated every 4 weeks. Patients with a performance status of 3 or 4 (seven patients) were started at 2 mg/m2 i.v. in the absence of grade 3 toxicity, the dose was escalated. Complete responses were seen in 36 patients (42.4%) and 35 patients had partial responses (41.2%) for an overall response rate of 83.6%. Median time to best response was 6.5 months. Eight patients had stable disease, and one patient had progressive disease. Three patients died early during the treatment phase and two were not evaluable due to treatment violations. Of seven patients with a CALGB performance status of 3 or 4, there were no CRs and only two PRs. Of 31 evaluable PS 0-2 patients who had previously achieved only stable disease on interferon, 13 had a CR on pentostatin and 12 had a PR. Based on PS 0-2 patients, the median follow-up is 44 months and 36 month remission duration and survival rates and 95% confidence intervals are 84% (CI 68-93%) and 91% (CI 81-96%). The 36 month survival rate was 29% (CI 10-58%) for PS 3-4 patients. Drug dosage was modified in 39 (51%) of 76 evaluable patients. Leukopenia and/or infection were the most frequent toxicities leading to a dose modification. Pentostatin is an effective agent and induces an excellent response in relapsed HCL patients previously treated with alpha-interferon (alpha-IFN).