2-Chlorodeoxyadenosine (2-CdA), a purine nucleoside, has been shown to be remarkably effective in the treatment of patients with hairy cell leukemia (HCL). We hereby report on the results achieved in 26 HCL patients treated with one single course of 2-CdA at a dose of 0.1 mg/kg daily for 7 days continuous infusion. Twenty-four were males and 2 females, with a median age of 56 years; all but 5 had been previously treated with Interferon-alpha (IFN-alpha). All cases are fully evaluable for their clinical and hematological response. Twenty of them (77%) achieved a complete remission and 6 a partial remission; two of the latter progressed after 6 and 12 months, respectively. The median duration of response was 13.8 months, ranging from 7 to 22 months from the end of therapy. Circulating hairy cells and spleen enlargement, when present, disappeared within 2 weeks after completing treatment; furthermore, a rapid normalization of soluble Interleukin-2 receptor serum levels was observed in all complete responders but one, and in 2 of the 6 partial responders. A significant lymphocytopenia was observed in almost all patients, whilst a severe neutropenia (< 500/microliters) was registered mainly in the 10 patients who had less than 1,000/microliters neutrophils when treatment was started; the hemoglobin and platelets were marginally affected in only a few cases. Eight of the 10 patients who developed severe neutropenia experienced fever; in 4 of them it was short-lived (24 hours) and apparently not infection-related, while the remaining 4 probably had an infection.(ABSTRACT TRUNCATED AT 250 WORDS)