The relationship between breastfeeding and cognitive development in the first 2 years of life was examined in a cohort of children being followed in a study of risk factors in development. A significant difference between bottlefed children, children breastfed less than or equal to 4 months and those breastfed greater than 4 months was found on the Mental Development Index of the Bayley Scales at ages 1 and 2 years, favoring the breastfed children. At age 6 months, the direction of the relationship was the same but did not reach significance. Supplementary regression analyses examining the strength of the relationship between duration of breastfeeding and cognitive development similarly showed a small but significant relationship between duration of breastfeeding and scores on the Bayley at 1 and 2 years. Alternative explanations for the results are discussed.