The detrimental effects of maternal drinking during pregnancy on fetal health have been documented. The consequences for infants of maternal drinking during breast-feeding are unknown, but research in animals suggests that the infant could be affected by exposure to alcohol through the mother's milk. In a study of 400 infants born to members of a health maintenance organization, we investigated the relation of the mother's use of alcohol during breast-feeding to the infant's development at one year of age. Mental development, as measured by the Bayley Mental Development Index (MDI), was unrelated to maternal drinking during breast-feeding. However, motor development, as measured by the Psychomotor Development Index (PDI), was significantly lower in infants exposed regularly to alcohol in breast milk (after alcohol exposure during gestation was controlled for), with a dose-response relation (P for linear trend, 0.006). The infants of breast-feeding mothers who had at least one drink daily had a mean PDI score of 98, whereas the infants exposed to less alcohol in breast milk had a mean PDI score of 103 (95 percent confidence interval for the difference of the two means, 1.2 to 9.8). The effect was more pronounced when mothers who supplemented breast-feeding with formula were excluded from the analysis. The association persisted even after we controlled for more than 100 potentially confounding variables, including smoking and other drug use during pregnancy and in the postpartum period. We conclude that ethanol ingested through breast milk has a slight but significant detrimental effect on motor development, but not mental development, in breast-fed infants.