The purine nucleoside analogues (PNAs), fludarabine (FA), 2-CdA (2-chlorodeoxyadenosine, 2-CdA) and pentostatin (2'-deoxycoformycin, DCF) represent a group of cytotoxic agents with high activity in lymphoid and myeloid malignancies. PNAs share similar chemical structure and mechanism of action. Several mechanisms could be responsible for their cytotoxicity both in proliferating and quiescent cells, such as inhibition of DNA synthesis, inhibition of DNA repair and accumulation of DNA strand breaks. Induction of apoptosis through the mitochondrial pathway, direct binding to apoptosome or modulation of p53 expression all lead to apoptosis, which is the main end-point of PNA action. However, individual PNAs exhibit significant differences, especially in their interaction with enzymes involved in adenosine and deoxyadenosine metabolism. Synergistic interactions between PNAs and other cytotoxic agents (alkylating agents, anthracycline antitumor antibiotics, cytarabine, monoclonal antibodies) have been demonstrated in both preclinical and clinical studies. PNAs are highly effective in chronic lymphoid leukemias and low grade B- and T-cell non-Hodgkin's lymphomas, including Waldenström's macroglobulinemia. DCF and 2-CdA are currently the drugs of choice in hairy cell leukemia. Moreover, clinical studies have confirmed the efficacy of PNAs alone or in combination protocols in the treatment of acute myeloid leukemia and myelodysplastic syndromes. Finally, PNAs, especially FA, play an important role in non-myeloablative conditioning regimens for allogenic stem cell transplantation in high-risk patients. The toxicity profiles of PNAs are similar for all agents and consist mainly of dose-limiting myelotoxicity and prolonged immunosuppression. Three other compounds: clofarabine, nelarabine and immucillin-H are currently being evaluated clinically.