An estimated one-third to one-half of vertical transmission of human immunodeficiency virus type 1 (HIV-1) worldwide is due to breastfeeding. The exact frequency of breast milk HIV-1 transmission is unknown, but it has been estimated to be 14% in the setting of established maternal HIV-1 infection and 29% in the setting of acute maternal infection. The timing of breast milk transmission during the course of lactation also remains unknown, but two studies have found an association between duration of breastfeeding and risk of infant infection. In one such study, prolonged breastfeeding for 15 months or longer was associated with a twofold increased transmission risk. Human immunodeficiency virus type 1 DNA can be detected in over 50% of breast milk samples and is correlated with CD4 depletion and vitamin A deficiency. The presence of breast milk HIV-1 provirus is associated with increased transmission risk. Many current intervention strategies to prevent vertical transmission of HIV-1 are aimed at in utero or perinatal transmission. In developing countries in which breastfeeding by HIV-1 infected women is recommended practice, additional intervention strategies to reduce breast milk transmission warrant evaluation.