Twelve evaluable patients with progressive hairy cell leukemia were treated with deoxycoformycin at a dose of 4 mg/m2 every 2 weeks. Five patients had not been splenectomized, and one had failed to respond to interferon-alpha. Complete remission, as defined by absence of hairy cells in the bone marrow and normalization of the peripheral blood and regression of splenomegaly, was obtained in 11 of 12 patients (92%). These patients have remained in unmaintained remission for 1+ to 13 months with an average of 7.5 months. Two of these patients had a bone marrow relapse at 8 and 12 months, respectively. During treatment the monocytopenia corrected, and, after complete remission was obtained, marrow was aspirable. Toxicity was mild and reversible. There were no significant infections associated with this treatment. It was of interest that we could treat two patients with creatinine clearance of 50 and 60 ml/min using lower doses (and 2-3 mg/m2) than our conventional therapy of 4 mg/m2 every 2 weeks. They obtained a complete remission after 6 and 10 treatments, respectively. Low-dose deoxycoformycin has proven to be an excellent treatment for hairy cell leukemia.