The importance of breast-feeding for infant and child health and survival in less developed countries has been the subject of a number of studies over the last 25 years.^1–9 However, the epidemic of AIDS in low-income countries, together with the discovery of an important risk of mother-to-child transmission of HIV-1 through breast-milk,¹⁰ has prompted renewed interest in this subject. Indeed, in order to weigh the mortality risks associated with different infant feeding practices against the risk of mother-to-child transmission of the virus, it is essential to have precise estimates of relative risk of death of non-breastfed infants within narrow age intervals.