Objective: To evaluate the efficacy and safety of zidovudine early in the treatment of human immunodeficiency virus type 1 (HIV) infection. Design: A double-blind, randomized, placebo-controlled trial with subject stratification by pretreatment CD4 T lymphocyte counts. Seting: Multicenter trial at AIDS Clinical Trials units. Subjects: Seven hundred eleven subjects with mildly symptomatic HIV infection. Intervention: Three hundred fifty-one subjects were assigned to placebo and 360 to zidovudine, 200 mg orally every 4 hours. The median duration of follow-up was 11 months. Measurements and main results: Fifty-one subjects developed the acquired immunodeficiency syndrome (AIDS), advanced AIDS-related complex, or death as a first critical event. For the stratum of subjects with more than 200 but less than 500 CD4 T lymphocytes/mm3 before treatment, 34 events occurred in placebo recipients and 12 in zidovudine recipients (P = 0.0002; relative risk [RR] estimate, 3.23 [95% CI, 1.67 to 6.24]). For the stratum of subjects with 500 to 799 CD4 T lymphocytes /mm3 before treatment, 2 events occurred in placebo recipients and 3 in zidovudine recipients. Candidiasis at study entry independently increased the risk for having an event (P = 0.005; RR estimate, 2.3 [95% CI, 1.29 to 4.12]); HIV antigenemia at study entry also increased this risk (P = 0.01; RR estimate, 2.1 [95% CI, 1.2 to 3.8]). Significant differences between the treatment groups in CD4 T-lymphocyte counts occurred in subjects with more than 200 but less than 500 CD4 T lymphocytes/mm3 after 4 weeks of therapy (P = 0.002). Differences persisted through week 52. Less prominent changes occurred in subjects with 500 or more CD4 T lymphocytes/mm3. Serum levels of HIV antigen decreased significantly in zidovudine recipients. Serious anemia and neutropenia occurred in 5% and 4% of zidovudine recipients, respectively, and in 0% and 1% of placebo recipients, respectively. Conclusion: Zidovudine delayed progression of HIV disease and produced little toxicity in subjects with mildly symptomatic HIV disease and less than 500 CD4 T lymphocytes/mm3.