Zidovudine (azidothymidine, Retrovir) and ddI (di-deoxy-inosine, Videx) interfere with the multiplication of HIV by incorporation into nascent DNA chains and interruption of the further linking of nucleotides. Zidovudine lowers early mortality in patients with Aids and pneumocystis carinii pneumonia. However, much of the effectiveness of zidovudine is lost later on; the average prolongation of life in treated patients is estimated to be about 1 year. About two thirds of patients with Aids can be treated with zidovudine; in the others, the drug is ineffective or contraindicated. Frequent blood counts are necessary to monitor myelotoxicity, even at relatively low doses of 500 mg/day. In contrast, zidovudine is well tolerated by asymptomatic patients with 200 to 500 CD4 lymphocytes/mm3, in whom it diminishes the incidence of Aids from about 7 to 3% during the first year of treatment, with less than 2% severe anemia or leukopenia. For patients who do not tolerate zidovudine, ddI is an alternative. It is not myelotoxic but can cause neuritis and pancreatitis, especially at doses in excess of 10 mg/kg/day. Although its antiviral effect is excellent both in vitro and in vivo, there is still a lack of firm data on its clinical value, such as the decrease in opportunistic infections and increase in survival.