Breast feeding was reported in 1992 by Lucas, et al. to provide advantages for the development of intelligence in children of low birth weight, possibly through nutrients or other biological factors found in human breast milk but not cow's milk. Research on breast feeding and intelligence in children of normal birth weight has yielded mixed results, probably because measurement of environmental influences has not been thorough and the range of intelligence components measured has been limited. Our research with 204 3-year-old children of normal birth weight included control measures for the environment and maternal intelligence (Hollings-head socioeconomic status, Home Observation for the Measured Environment, Shipley) and two measures of childhood intelligence (Stanford-Binet Fourth Edition and Peabody Picture Vocabulary Test-Revised). Controlling for environmental variables and maternal intelligence, initiation of breast feeding predicted scores on intelligence tests at age three. Breast feeding was associated with 4.6-point higher mean in children's intelligence.