Sixty-six mothers of 66 breast-fed infants with infantile colic were put on a diet free from cow's milk. The colic disappeared in 35 infants; it reappeared on at least two challenges (cow's milk to mother) in 23 infants (35%). A double-blind crossover trial with cow's milk whey protein was performed in 16 of these 23 mothers and infants. Six infants had to be taken out of the study for various reasons; of the remaining ten infants, nine reacted with colic after their mothers' intake of whey protein-containing capsules. Sequential analysis showed a high correlation between infantile colic in breast-fed infants and their mothers' consumption of cow's milk protein. A diet free of cow's milk is suggested for the mothers as a first trial of treatment of infantile colic in breast-fed infants.