We did a cost-effectiveness analysis of zidovudine therapy for asymptomatic patients with human immunodeficiency virus (HIV) infection. The incremental direct medical costs of zidovudine therapy were determined, and data on the effects of therapy were derived from the report of the Acquired Immunodeficiency Syndrome (AIDS) Clinical Trials Group Protocol 019. Zidovudine therapy has an annual incremental cost of $2653 per person. The cost per year of life saved ranges between $6553 and $70 526, depending on which epidemiologic model of potential long-term zidovudine effect is used (in sensitivity analyses, the cost per year of life saved ranges between $2649 and $250 546). Although expensive, zidovudine therapy has the potential to yield a substantial survival benefit to patients. The cost effectiveness of zidovudine therapy compares favorably with that of other common medical therapies.