A prospective study was conducted in a level II maternity unit to investigate the incidence of hyperbilirubinemia in healthy, term, breast-fed and formula-fed infants. Serum bilirubin levels were determined for 176 breast-red and 164 formula-fed infants in cord blood and on days 1, 2, 3 and 5 after birth. The mean total bilirubin levels were significantly higher on each postnatal day in the breast-fed infants, as was the proportion of infants with peak levels above 12 mg/dl (205 mumol/l; 28% v. 6%). The breast-fed infants also had significantly higher proportional weight losses on each postnatal day than the formula-fed infants. However, there was no correlation between the cumulative weight loss on day 3 and bilirubin levels on the same day with either feeding regimen. None of the infants required an exchange transfusion or prolonged care in hospital for hyperbilirubinemia.