We examined the relationship between sex and the risk of intrauterine, intrapartum and postnatal HIV transmission among 4495 infants born to HIV-infected mothers in Harare, Zimbabwe. Intrauterine transmission was 8.6%, and consistent with other studies was higher among girl than boy infants (AOR 1.53; 95% CI 1.23-1.91). Unlike previous studies, we observed no independent effect of infant sex on intrapartum or breastfeeding-associated HIV transmission. Sex-specific postnatal prevention strategies are not warranted in this population.