Two men with advanced but previously untreated B cell hairy-cell leukemia were treated with low doses of pentostatin (2'-deoxycoformycin) in intermittent courses. There was prompt clearance of hairy cells from the blood, regression of splenomegaly and lymphadenopathy, and correction of anemia, thrombocytopenia, and granulocytopenia. Side effects were tolerable and myelosuppression was not observed. Both patients achieved complete remission documented by bone marrow aspiration and biopsy and radionuclide scans of liver and spleen. They remain in complete remission nine and six months, respectively, after their last treatment. Pentostatin (Warner-Lambert, Ann Arbor, Mich) is highly active in hairy-cell leukemia and merits more extensive evaluation in this disease. A woman with hairy-cell leukemia has begun treatment with pentostatin, and at ten weeks there is disappearance of gross splenomegaly and clearance of hairy cells from the blood. Bone marrow studies have not yet been repeated.