The prevalence of colic with respect to the type of milk feeding in the first 17 weeks of life was assessed by questioning the parents of 964 healthy infants aged 2 to 52 weeks. There was a similar prevalence of colic in infants fed human milk (20%), formula (19%), and formula-supplemented human milk (21%). Intestinal damage, determined by measuring random fecal alpha 1-antitrypsin concentrations in 206 infants aged 2 to 17 weeks and fecal hemoglobin concentrations in 200 of these, was not more likely in infants with colic at the time of study. The occurrence of adverse reactions at the time of introduction of fresh whole cow's milk into the diet of previously colicky infants was uncommon. Our results suggest that dietary protein hypersensitivity is probably not the cause of colic in most healthy young infants.