18 mothers of 19 breast-fed infants with infantile colic were put on a diet free of cow's-milk protein. The colic disappeared promptly from 13; in 12, it reappeared on at least two further indirect challenges (in the form of a diet containing cow's milk to the mother). Most infants became symptom-free at age 2 to 4 months; at 4 months, only 4 reacted with colic when the mother took cow's milk. 5 infants were directly challenged with cow's milk; 4 reacted promptly with colic. Other signs of intolerance to cow's-milk protein developed in 3 infants during weaning. The treatment of infantile colic in breast-fed infants by a diet free of cow's milk for the mother appears worthwhile.