Ten patients with progressive hairy cell leukemia were treated with 2'deoxycoformycin (dCF) by intravenous bolus (4 mg/m2) given every other week. All ten patients are evaluable for response and nine of the ten patients have achieved a complete remission. In addition to clearing of hairy cells from the bone marrow, eight patients had resolution of their monocytopenia. Seven of the nine patients remain in unmaintained remission with a median duration of 6.2 months. Two patients have had relapse in the bone marrow alone and continue to have normal peripheral blood counts. They are being followed without treatment. Toxicity was minimal at this low dose with one patient having a mild reversible reduction in creatinine clearance. Four other patients had reversible neutropenia. There were no significant infections associated with treatment. Low-dose deoxycoformycin administered intravenously every other week represents an extremely effective treatment for hairy cell leukemia.