One-half of the healthy newborn infants at a rural medical center were initially breast fed; the proportion declined to 4% by one year of age. Breast-feeding was associated with significantly less illnes during the first year, especially if continued beyond 41/2 months of age. Breat-fedding was associated with a higher level of parental education and, by inference, higher socioeconomic status. The health advantage of breast-feeding was still evident after controlling for parental educational status. In better educated families the difference in significant illness between infants who were artifically fed and those who were breast fed for prolonged periods of time was two- to threefold.