We examined the prospective associations between breast milk concentrations of erythropoietin, a factor with trophic effects on infant gut epithelia, and the risk of mother-to-child transmission (MTCT) of human immunodeficiency virus (HIV) through breast-feeding in a study of 59 MTCT cases and 116 controls nested within a cohort of antiretroviral-naive HIV-infected Tanzanian women. Controls were matched to cases on the basis of the time from birth when the breast milk sample was collected. The risk of MTCT was inversely related to breast milk erythropoietin concentration (adjusted odds ratio for highest vs lowest erythropoietin concentration tertile, 0.34 [95% confidence interval, 0.14-0.82]; P = .02). These results suggest a protective effect of breast milk erythropoietin against MTCT.