The results of a Phase III, multicenter, double-blind, randomized, placebo-controlled clinical trial to evaluate the efficacy, safety, and tolerance of zidovudine for the prevention of HIV transmission from infected pregnant women to their infants (ACTG protocol 076) provided for the first time proof of the concept that a preventive intervention can reduce vertical HIV transmission (47). Based on analysis of data for 364 evaluable births, zidovudine (ZDV or AZT) treatment according to the regimen employed in ACTG 076 appeared to reduce the risk of HIV transmission by two thirds, from 25.5 percent to 8.3 percent. Eligible subjects were HIV-infected pregnant women who had received no antiretroviral therapy during their current pregnancy, who had no maternal clinical indications for antiretroviral therapy, and who had CD4+ T-lymphocyte counts above 200 per microliter at study entry.