This study examined the relationship between fecal output and neonatal jaundice in infants exclusively fed either human milk or one of three infant formulas (whey predominant, Enfamil; casein predominant, 3305H; and casein hydrolysate, Nutramigen; all from Mead Johnson, Evansville, IN). Stool output was quantitated during the first 3 weeks of life. Jaundice was assessed by measuring serum bilirubin level and transcutaneous jaundice index. In general, after the fourth day, breast-fed infants produced lower-weight individual wet and dry stools than formula-fed infants. Cumulative wet and dry stool output was also lowest in the breast-fed infants during this time. After the first week, breastfed infants had a higher stooling frequency than formula-fed infants. The jaundice indexes of the four groups differed significantly on all days after day 3, with highest levels in breast-fed infants and lowest levels, for unknown reasons, in those fed casein hydrolysate. The jaundice index of those fed casein hydrolysate was significantly lower than that of the other formula-fed infants on days 10-18. In the breast-fed group the decrease from day 3 to day 21 in both serum bilirubin level and the jaundice index was positively correlated with both the 21-day total wet and total dry cumulative stool weights. It is concluded that the quantity of stool excreted is related to decreases in serum bilirubin levels in infants fed human milk.