-DESCRIPTION (provided by applicant): It is well established that breast fed infants of HIV-infected mothers can acquire HIV infection through breast milk. In low resource settings, where HIV predominates, breastfeeding (BF) cannot simply be replaced by formula since it is often unaffordable and unsafe. Even with the potential for more widespread access to antiretroviral drug treatment, questions about the risks and benefits of different infant feeding modalities for HIV-positive women in low resource settings remain and are not being addressed by other studies. With this application we request 2 years of funding to complete a prospective cohort study testing the safety and efficacy of short duration (4 months) exclusive breast feeding (EBF) to minimize risks of mother to child HIV transmission (MTCT) without increasing < 2-year mortality among uninfected infants. Full accrual of 1435 HIV-infected pregnant women and their infants has been achieved at two urban primary health care clinics in Lusaka, Zambia. All women are counseled to exclusively breastfeed to 4 mo. Half are randomized to a counseling program encouraging abrupt cessation of BF at 4 mo, and half to a program that encourages continued EBF through 6 mo with the introduction of weaning foods and continued BF for a duration of the mother's own choice. Mother infant pairs are followed for 2 years with regular medical histories, physical exams and clinical sampling. The primary objective is to compare 24-mo HIV-free survival and to quantify the magnitude of postnatal MTCT due to prolonged BF, and the excess risks of under-2 year mortality due to early cessation of BF. The final months of followup are critical to quantify the primary study outcomes of late postnatal MTCT and weaning associated child morbidity and mortality. The overall study is evaluating an inexpensive and potentially sustainable public health intervention to reduce HIV transmission through breast feeding while preserving benefits of breast feeding for other aspects of child health in a very low resource setting. The question of the relative risks and benefits of early cessation of breastfeeding remains crucially relevant in an era where antiretroviral drug treatment is poised to become more widely available.