Twenty-six patients with hairy cell leukemia (HCL) were treated with 2-chlorodeoxyadenosine (2-CdA), a purine analogue resistant to adenosine deaminase, at 0.1 mg/kg/d for 7 days by continuous intravenous infusion. Fifteen patients were previously untreated, while 11 patients had received prior treatment with splenectomy alone (three patients), interferon alpha alone (four), splenectomy, then interferon alpha (two), or splenectomy, interferon alpha, then 2-deoxycoformycin (2-DCF) (two). Sixteen (80%) of 20 patients evaluable at 3 months achieved complete remission (CR), and four (20%) achieved partial remission (PR) following a single cycle of therapy. All four patients in PR had complete recovery of their peripheral blood counts (except one patient whose platelet count remained 84,000/microL), but had residual HCL in the bone marrow (three patients) or residual splenomegaly (one). Patients with bulky adenopathy, massive splenomegaly, and severe pancytopenia responded as well as those with only modest marrow involvement. The three patients with residual marrow disease received a second cycle of 2-CdA, and two have attained CR. Therefore, 18 of 20 (90%) achieved CR with either one or two cycles of therapy. No patient achieving CR has relapsed at a median follow-up of 12 (+/- 2.1) months. Toxicities included myelosuppression and culture-negative fever. A community-acquired pneumonia was the only infectious complication. Since a single cycle of 2-CdA induces sustained CR in the vast majority of patients with minimal toxicity, this agent is emerging as the treatment of choice for all patients with HCL.