Although 2-chlorodeoxyadenosine (2-CdA) is effective in inducing complete remissions (CRs) in the majority of patients with hairy cell leukemia (HCL), neither the actual relapse rate, the clinical factors that may predict relapse, the long-term outcome, nor the response rate to re-treatment at relapse has been clearly determined. Fifty-two consecutive patients with previously untreated or treated HCL were treated with 2-CdA at a dose of 0.1 mg/kg/d by continuous intravenous infusion for 7 days. Of 50 assessable patients, 40 (80%) achieved CR, and 9 (18%) achieved partial remission (PR). A total of 7 patients (14%) have relapsed, at a median duration of 24 months (range, 12 to 44). Of the 7 relapsed patients, 5 were re-treated with a second cycle of 2-CdA; 2 achieved a second CR and 3 attained a PR. The progression-free survival (PFS) rate is 72% at 4 years for all 52 patients and 83% for patients achieving CR. The overall survival (OS) rate is 86% at 4 years. Only prior therapy was predictive of relapse. The majority of patients achieve durable CRs with a single cycle of 2-CdA. The relapse rate is low and the long-term prognosis is excellent. The few patients who relapse can attain second remissions after re-treatment with 2-CdA.