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To examine predictors of vertical transmission of HIV-1 in Dar-es-Salaam, Tanzania.Observational design.Consenting HIV-1-infected pregnant women (n = 1078) were enrolled in a trial to examine the role of vitamin supplements. Intrauterine HIV-1 infection (HIV-positive at birth); intrapartum and early breastfeeding transmission (HIV-positive at 6 weeks among those uninfected at birth) were defined using the PCR.Of 734 infants who had a specimen taken at birth, 62 were HIV positive [8.4%; 95% confidence interval (CI),6.4–10.5%], whereas 59 infants were positive among 367 infants who were uninfected at birth and were retested at 6 weeks (16.1%; 95%CI, 12.3–19.8%). In multivariate analyses, maternal CD4 cell count, viral load, and clinical stage were significant predictors of both definitions of transmission. Viral load of 50 000 copies/ml or more at delivery was associated with a 4.21-fold increase in risk of intrapartum and early breastfeeding transmission (95%CI, 1.59–11.13;P = 0.004). Babies who were HIV negative at birth and born before 34 weeks of gestation were 2.19 times more likely to become infected during intrapartum and early breastfeeding periods compared with those born after 37 weeks (95%CI, 1.19–4.04;P = 0.01). Gonorrhea at baseline was related to intrauterine transmission [multivariate risk ratio (RR), 5.50; 95%CI, 2.04–14.81;P < 0.001] but not intrapartum and early breastfeeding transmission. Signs of lower genital infections at or after enrollment were also associated with transmission.Reducing prematurity, rate of HIV disease progression, and maternal viral load at or after delivery could help to reduce vertical transmission. Treatment of sexually transmitted infections at onset of prenatal care, about 20 weeks on average, was inadequate for prevention of transmission. Whether sustained clearance of lower genital tract infections result in reduced transmission remains to be determined.