In a prospective study of 1019 infant and mother pairs, 268 infants suffered with 'infantile colic'. Significantly more of these were breast fed, supporting the proposed theory of a colic-producing factor in breast milk. Social class and maternal education were not related to the incidence of colic. Seventy-two percent of our infants with colic suffered from classical evening colic. The symptoms did not disappear from all the infants by 3 months - over 38% continued to have symptoms after 3 1/2 months of age. Treatment is limited, but merbentyl helped to relieve symptoms to a varying degree in over 60% of the infants who tried it. The family relationships are often strained during this period and this appears to persist when at one year of age significantly more of these infants were described as demanding, miserable and bad-tempered by their mothers.