Background: Pentostatin (Nipent®), a potent inhibitor of adenosine deaminase, has proven its efficacy in the treatment of hairy-cell leukemia. However, there are only few reports on the pentostatin treatment of prolymphocytic leukemia (PLL), a disease with generally poor response to conventional chemotherapy consisting of anthracycline-containing combinations. Case Report: We report on a 64-year-old female patient in whom PLL was diagnosed in September 1995. The leukemic bone marrow infiltration at diagnosis was 90%. The initial hy-perleukocytosis of 850,000/μlwas reduced by leukapheresis to 600,000 leukocytes/μl. Chemotherapy with Vincristine, Adria-mycin and cortisone reduced the leukocytes to 150,000/μl; a second course of the same chemotherapy did not further decrease the leukocyte counts. In contrast, 3 doses of pentostatin induced a partial remission within 4 weeks with normal peripheral blood cell counts and with a leukemic bone marrow infiltration of only 25%. Pentostatin was tolerated without any side effects. In spite of continued pentostatin administration for 2 months, the patient relapsed and died from tumor progress. Conclusions: Pentostatin may be efficacious in the treatment of PLL, even in conventionally pretreated patients. Pentostatin treatment alone, however, may not be sufficient to significantly alter the natural course of the PLL, but requires additional therapeutic age