Fecal bile acid patterns and concentrations have been determined for 28 infants who were followed from average ages of 3–11 months. Half were solely breast-fed and half were solely formula-fed at the beginning of the study. Breast-fed infants were found to have significantly (p < 0.05) lower concentrations of cholic acid than the formula-fed group, up to an average age of 5 months. Concentrations of deoxycholic and lithocholic acids were directionally lower in breast-fed infants at all ages. Concentrations of chenodeoxycholic acid were similar for both groups throughout the study. At the end of the study, breast-fed infants were excreting 17% of their total bile acids in the form of secondary acids, compared to 33% for formula-fed infants. This pattern persisted long after the infants began weaning. Formula-fed infants were found to have lithocholic acid in their stools at a significantly (p < 0.05) earlier age than breast-fed infants. Appearance of deoxycholic acid was at similar ages for both groups. Both of these secondary acids were found to occur at much younger ages (approximately 2 months) than has been previously reported. These observed differences are attributed to the distinct intestinal microbial populations encouraged by the different diets.