Fludarabine is a prodrug that must enter cells and be phosphorylated to the nucleoside triphosphate, F-ara-ATP, to elicit biological activity. F-ara-ATP serves as an inhibitory alternative substrate in several key processes involved in DNA synthesis. The enzymes required in DNA synthesis and affected by F-ara-ATP are ribonucleotide reductase, DNA primase, DNA polymerases, 3'-5' exonuclease activity of DNA polymerases delta and epsilon, and DNA ligase I. The action of fludarabine on DNA replication provides a compelling rationale to use this agent for leukemias where target cells are actively synthesizing DNA, for example acute myelogenous leukemia. Additionally, the role of F-ara-ATP to potentiate the activity of deoxycytidine kinase makes it an appropriate candidate to use in combination with other nucleoside analogs which require deoxycytidine kinase for their activation. The present article reviews the effect of fludarabine on enzymes involved in DNA synthesis and the role of fludarabine in combination with arabinosylcytosine for the treatment of diseases other than indolent leukemias.