OBJECTIVE: To describe risk factors for infantile colic. DESIGN: Questionnaire administered by health visitors. SETTING: Sheffield. SUBJECTS: Mothers of 76,747 infants born between 1 August 1975 and 31 May 1988, interviewed when the infant was 1 month old. MAIN OUTCOME MEASURES: Reporting of infantile colic and its duration; weight of infant leeding, state of the home, socioeconomic characteristics of the parents, parents' age, and mother's parity. RESULTS: The odds of reporting infantile colic were increased with breast feeding (odds ratio of breast v bottle feeding 1.35 (95% confidence interval 1.28 to 1.43)), increasing parental age, lower parity, increasing parental age at leaving full time education, and more affluent homes and districts of residence. In a logistic regression analysis, mother's age and parity and socioeconomic factors remained the most important risk factors for the reporting of infantile colic (each P < 0.005), and the effect of breast feeding was attenuated (odds ratio of breast v bottle feeding 1.09 (1.02 to 1.15)). CONCLUSION: At a population level, dietary factors contribute little to mothers' reporting of infantile colic, and dietary change should not be the primary intervention.