Zidovudine, the first widely used antiretroviral agent, prevents replication of the human immunodeficiency virus (HIV) by inhibiting reverse transcriptase. Its use in patients with acquired immunodeficiency syndrome slows progression of the disease and prolongs survival. Zidovudine also significantly reduces the rate of progression to AIDS in adults with asymptomatic HIV infection and CD4 T-lymphocyte counts below 500 per mm3. The major toxicity of the drug is bone marrow suppression resulting in anemia or granulocytopenia, or both. Recently, lower doses have been shown to be effective and are associated with less toxicity.