A multicenter randomized, double blind, placebo-controlled clinical trial was conducted to evaluate the effectiveness of a short course of oral zidovudine (ZDV) treatment in HIV-1 infected pregnant women, starting at 38 weeks of gestation plus ZDV infusion during labor until delivery, to reduce HIV-1 vertical transmission in non-breast fed infants. One hundred and eighty two asymptomatic antiretroviral naïve HIV-1 infected pregnant women were enrolled. Each patient was randomly allocated into either the ZDV or placebo group. The ZDV group received 250 mg ZDV orally twice a day initiated at 38 weeks' gestation until the onset of labor. During the intrapartum period, ZDV infusion at the rate of 2 mg/kg was administered within the first hour and then continuously infused at the rate of 1 mg/kg/h until delivery. The placebo group received an identical capsule during pregnancy and normal saline infusion during labor until delivery. HIV-1 transmission was documented by nested polymerase chain reaction in infants at birth and at 1, 3 and, 6 months of age. The estimated HIV-1 vertical transmission rate was 14.9 per cent (95% CI = 11.1 to 18.7) and 16.3 per cent (95% CI = 12.3 to 20.9) in ZDV and placebo group, respectively (p > 0.05). The short course ZDV in antiretroviral naïve pregnant women initiated at 38 weeks' gestation plus intrapartum ZDV infusion without treatment in the infants was not effective to prevent HIV-1 vertical transmission.