A prospective study of the bowel habits of 240 infants aged 2-20 weeks was performed. Half of the infants were breast fed and half were fed cow's milk formula (CMF). Breast-fed infants had a higher mean frequency of defaecation than did CMF-fed infants at 2, 4 and 8 weeks (p less than 0.02). By 16 weeks the mean frequency of bowel actions per day of infants of both feeding groups was 2. With increasing age, infants produced fewer stools of greater firmness (p less than 0.05) though the breast fed tended to pass larger, softer stools than did the CMF-fed infants until the introduction of weaning foods (p less than 0.05). Stool colour was uniformly yellow in both feeding groups until the introduction of weaning foods, when it changed to brown. Weaning foods were introduced at an earlier age in the CMF-fed infants (p less than 0.05), though by 20 weeks 93% of all infants had received some. These data define the normal range of the bowel habits of milk-fed infants and may reflect differences in gastrointestinal function between those fed on the breast and with CMF.