To review the structure, mechanism of action, pharmacologic features, and clinical trial results of the newer purine analog, 2-chlorodeoxyadenosine (2-CdA). English-language medical literature review of more than 70 articles. 2-Chlorodeoxyadenosine is unique compared with traditional antimetabolite drugs in that it is equally active against dividing and resting lymphocytes, which may be especially important in indolent lymphoid malignancies, such as chronic lymphocytic leukemia, because most cells in these disorders are in the resting phase. In patients with alkylator-refractory chronic lymphocytic leukemia who were treated with 2-CdA, 44% achieved a response (4% complete responses, 40% partial responses), and 54%, scored as nonresponders, had a sustained reduction in their peripheral lymphocytosis. Patients with untreated chronic lymphocytic leukemia had an 85% response rate (25% complete responses, 60% partial responses). Patients with previously treated low-grade lymphoma achieved an overall response rate of 43%. The most striking clinical effects of this drug have been seen in hairy cell leukemia, in which a single course of therapy induces complete remissions in 85% of partial remissions in 12%. Activity has also been shown in cutaneous T-cell lymphoma and the myeloid leukemias. 2-Chlorodeoxyadenosine is a newer purine analog with potent activity in the treatment of indolent lymphoproliferative diseases and illustrates the model for rational drug development.