One thousand four hundred and fifty-four consecutive full-term neonates, delivered over a period extending from February 1, 1991 to January 31, 1992 were prospectively studied during the first days of life to determine if breast-feeding and hyperbilirubinemia were related. Six hundred and five infants were exclusively breast-fed on demand, 623 received both breast- and formula-feeding, and 226 were exclusively formula-fed. Of the 1,454 newborns studied, 70 (4.8%) presented with a bilirubin serum concentration > 12.9 mg/dl (4.62% in the breast-fed group, 5.45% in the group fed with mother's milk with supplements, and 3.54% in the artificial formula group; the differences were not significant). Percent mean weight losses on the 3rd and 5th days were not significantly different. Babies breast-fed on demand seem to have a low incidence of hyperbilirubinemia similar to that found in formula-fed neonates.