Although vertical transmission of HIV-1 can occur through breast-feeding, little is known about the effect of colostrum, duration of breast-feeding, mixing feeding, and nipple pathology. We used retrospective cohort data to examine the association between breast-feeding-related factors and transmission of HIV-1 from mother to child in São Paulo State, Brazil. Information on maternal and postnatal factors was collected by medical record review and interview. Infection status was determined for 434 children by anti-HIV-1 tests performed beyond 18 months of age or diagnosis of AIDS at any age. Among 168 breast-fed children, the risk of transmission of HIV-1 was 21%, compared with 13% (p = .01) among 264 children artificially fed. Breast-feeding was independently and significantly associated with mother-to-child transmission of HIV-1 after controlling for stage of maternal HIV-1 disease (odds ratio [OR] = 2.2; 95% confidence interval [CI], 1.3-3.8). A trend was shown toward an increased risk of transmission with longer duration of breast-feeding, a history of bleeding nipples, and introduction of other liquid food before weaning, but these associations were not statistically significant. History of colostrum intake or cracked nipples without bleeding were not associated with transmission. Most of the women who breast-fed were unaware of their HIV-1 infection status at the time of delivery. Avoidance of mixed feeding and withholding of breast-feeding in the presence of bleeding nipples should be considered in further research as strategies to reduce postnatal transmission of HIV-1 in settings in which safe and sustainable alternatives for breast-feeding are not yet available.