Six patients with hairy cell leukemia have been treated with 2-chloro-2'-deoxyadenosine in a dose 0.05 mg/kg/daily in 2 h intravenous infusions during 7 consecutive days. Two patients underwent splenectomy with no clinical or hematologic improvement. Complete remission has been achieved in 4 patients and partial remission in one. 2-CdA was not effective in a patient with HCL-variant. The longest unmaintained CR lasted 11 months. The drug was well tolerated. Only slight side effects were observed. The drug exerted, however, myelosuppressive action resulting in cytopenias in the peripheral blood. Clinical consequences of this activity require further investigation. 2-CdA seems to be a drug of choice in the management of HCL.