The nucleoside analogues, 3'-azido-3'-deoxythymidine (AZT, zidovudine), 2',3'-dideoxyinosine (ddI, didanosine) and 2',3'-dideoxycytidine (ddC, zalcitabine), used in the treatment of human immunodeficiency virus (HIV) infection, have been associated with a number of dose-limiting toxicities in clinical studies. These include myelotoxicity (AZT), myopathy (AZT), peripheral neuropathy (ddC, ddI) and pancreatitis (ddI). Myopathy, peripheral neuropathy and pancreatitis are also observed in HIV-infected patients who have not been treated with the nucleoside analogues. Thus, nucleoside analogue toxicity can be confused with the adverse effects of HIV infection. Animal models exist for some, but not all, aspects of nucleoside analogue-related toxicity. In vitro studies have been used extensively to elucidate the mechanisms of nucleoside analogue toxicity. Cellular purine and pyrimidine metabolizing enzymes phosphorylate the analogues, which can then interact with DNA polymerases. Inhibition of one of these, HIV reverse transcriptase, is responsible for the antiviral activity of the nucleoside analogues. Toxicity is caused by inhibition of nuclear or mitochondrial DNA polymerases (or both) and by chain termination of replicating DNA at the point of insertion of the nucleoside analogue. The different toxicities observed in the case of each nucleoside analogue are most likely explained by different affinities for each of the cellular DNA polymerases.