MOTHER-TO-CHILD TRANS- mission of human immu- nodeficiency virus (HIV) can occur in utero, intra- partum, and postnatally.1,2 Postnatal HIV transmission through HIV- contaminated breast milk is of particu- lar concern in many developing coun- tries, where HIV infection in women is common and breastfeeding is almost universally practiced. Transmission of HIV through breast milk has been docu- mented in many studies,3-12 and HIV has been found in breast milk samples of HIV-infected women.13-16 Ascertaining the transmission risk of HIV at different times during the breast- feeding period has become particu- larly important, because it has re- cently been shown that in utero and intrapartum transmission can be de- creased by approximately 50% when short-course, oral antiretroviral therapy is used during pregnancy through la- 17 For editorial comment see p 781. 744 JAMA, August 25, 1999—Vol 282, No. 8 In breastfeeding populations, however, any decrease in in utero and intrapartum transmission of HIV achieved through such regimens or other methods of prevention will re- sult in a larger number of infants, who, though uninfected at birth, become ex- posed to HIV through breast milk. In this study, we investigated the risk of HIV transmission through breast- feeding in an urban setting in Malawi, where HIV prevalence in nursing women is approximately 30%, and breastfeeding is the recommended method of infant feeding. A revised statement in 1998 by the Joint United Nations Programme on HIV/AIDS18 rec- ommended that women be offered HIV counseling and testing, that they be in- formed of risks and benefits of breast- feeding if the mother is HIV-infected, and that they make a decision that takes into account their individual and fam- bor.