A case-control study was conducted to study the association between breast-feeding and inguinal hernia. The case group was significantly less often breast fed than control subjects (odds ratio, 0.49; 95% confidence interval, 0.29 to 0.83) and exclusive breast-feeding was associated with a significant dose-response risk reduction. The association was not confounded by birth weight, maternal education, type of birth, number of other children in the family, or gender. Breast-feeding may represent a protective factor against inguinal hernia.