This review discusses the pharmacokinetics, mechanism of action, clinical use, toxicities, drug interactions, and possible approaches for therapeutic monitoring of azathioprine (AZA). The drug has been used extensively in posttransplant immunosuppressive protocols. Its therapeutic use is hampered by the development of toxicities, however, especially leukopenia, which is a common criterion for dosage adjustment. Azathioprine is rapidly converted in the liver and erythrocytes to 6-mercaptopurine (6MP), which is eventually metabolized to inactive 6-thiouric acid (6TU). The terminal half-lives of AZA and 6MP are 50 and 74 minutes, respectively. While renal dysfunction does not alter the disposition of AZA, hepatic insufficiency attenuates the pharmacologic activity. Immunosuppression depends on the formation of active intracellular thiopurine ribonucleotides, although AZA itself may block antigen recognition. Individualization of AZA regimens by determining tissue concentrations of thioguanine nucleotides, and plasma concentrations of AZA, 6MP, or 6TU may improve the risk:benefit ratio.