Cladribine (2-chlorodeoxyadenosine: 2-CdA), a purine nucleoside analog, is a safe and very effective treatment for patients with hairy cell leukemia (HCL). 2-CdA was approved in USA and Japan, and is first-choice drug against symptomatic HCL. It is administered at a dose of 0.09 mg/kg daily as a continuous intravenous infusion over 7 days. Structure, mechanism of action, clinical data of efficacy and safety of 2-CdA, and indications for therapy for HCL are reviewed.