To assess the rate of excretion of bilirubin in the stools and its effects on serum bilirubin concentrations, we studied 24 breast-fed and 13 bottle-fed infants during the first 3 days after birth. Bottle-fed infants passed significantly more stool (3-day totals, 82 vs 58 gm, P less than 0.001), excreted more bilirubin (3-day totals, 23.8 vs 15.7 mg, P less than 0.05), and had lower serum bilirubin values (day 3, 6.8 vs 9.5 mg/dl, P less than 0.02). Among the breast-fed infants, greater stool output was associated with greater fecal bilirubin excretion (r = 0.56, P less than 0.05) and lower serum bilirubin concentrations (r = 0.66, P less than 0.001). Our data suggest that hyperbilirubinemia in breast-fed infants may be related to a delay in bilirubin clearance resulting from low stool output.