Because breast milk contains small quantities of thyroid hormones not found in commercial formula preparations, it was hypothesized that breast feeding may provide some protective benefit to the hypothyroid infant before medical treatment is begun. Of 108 children with congenital hypothyroidism, breast-fed children had higher thyroid hormone levels than formula-fed children at 1 and 2 months of age, but not later. The intellectual characteristics of the children were evaluated at 1, 3, 5, and 6 years of age. Breast-fed children scored higher in social and fine motor development at 1 year of age and in verbal learning at age 5, when controlling for parent intelligence quotient, socioeconomic status, and hormone dose level. Children with ectopic glands showed the largest benefit of breast feeding, particularly at age 3. These results signify a small protective benefit of breast feeding. (37 references) (JDD)