In only five years, antiretroviral therapy progressed from promising in vitro results with a newly recognized virus (HIV) to the standard practice of primary care applicable to hundreds of thousands of patients. With AZT (zidovudine) we have learned that we can prolong survival, reduce morbidity, and delay the progression of asymptomatic infection to disease. We have also learned that doses of AZT lower than those used in the original studies are as effective but less toxic. Additional compounds are under evaluation and more are needed to provide regimens, probably as combinations of drugs, that can be administered chronically with more efficacy, less toxicity, and diminished likelihood to select for drug-resistant virus.