Thirty seven hairy cell leukemia (HCL) patients, 35 males and 2 females with a median age of 53 years, were treated with a single course of 2-Chlorodeoxyadenosine (2-CdA) at a dose of 0.1 mg/kg daily for 7 days by continuous infusion. Twenty nine (78%) achieved a complete remission (CR) and 8 (22%) a partial remission (PR); four of the latter progressed after 6, 12, 18 and 24 months. All have been retreated with 2-CdA and 2 achieved a CR, 1 a PR and the last one is not yet evaluable. The overall median duration of response was 18 months, ranging from 4 to 30 months from the end of therapy. Circulating hairy cells and spleen enlargement, when present, disappeared within 2 weeks after completing treatment. A significant neutropenia was observed in almost all patients mainly in those who had less than 1,000/microliters neutrophils when treatment was started, together with a significant lymphocytopenia which lasted for more than 12 months. The hemoglobin and platelet levels were marginally affected. Fever was observed in 14 patients; in 8 of them it was short-lived (< or = 48 hours) and apparently not infection-related, while in the remaining 6 it was attributed to infection. Clinical tolerance was very good and none of the patients complained of nausea, vomiting or hair loss. In conclusion, our study confirms the efficacy of 2-CdA in HCL, including patients who progressed after treatment with 2-CdA.