SummaryTwo independent measures of human immunodeficiency virus type 1 (HIV-1) infection, virus isolation, and serum levels of p24 antigen were evaluated in a double-blind randomized clinical trial of the safety and efficacy of a nucleoside analogue, 3'-azido-3'-deoxythymidine (AZT) versus placebo in a single center. Pretreatment studies from 38 AIDS and AIDS-related complex (ARC) patients were comparably positive for virus isolation from their lymphocytes; all patients were qualitatively virus positive. Before AZT treatment, there was significantly decreased virus recovery in patients with higher numbers of CD4-positive lymphocytes. Within 1 month of AZT therapy, the time in culture required to register virus positivity was increased markedly in the AZT-treated group, and over the following several months progressive diminution in virus recovery was noted. Similar changes were not seen in patients concurrently receiving placebo treatment. Before treatment, 16 of 20 and 12 of 16 patients in the AZT and placebo groups, respectively, were p24 antigen positive. Marked reduction in serum p24 levels were noted in 11 of 16 (69%) of the p24 antigen-positive AZT-treated patients compared to 3 of 12 (25%) of the p24 antigen-positive placebo-treated patients (p = 0.02). There was a marked virologic response in 14 of 20 (70%) of the AZT-treated patients compared to 4 of 18 (22%) placebo-treated patients (p = 0.004). A higher frequency of positive clinical and immunological effects also were noted in the AZT-treated patients relative to placebo-treated patients (p = 0.02 and p = 0.06, respectively). These data from a concurrently randomized study support the hypothesis that the observed clinical efficacy of AZT relative to placebo is associated with an antiviral effect and suggest that virologic endpoints may serve as early surrogates of clinical outcome in AIDS antiviral chemotherapy trials.