The objective of this study was to analyze the effects of breast-feeding on the intellectual development of the infant, controlling for possible confounding factors. A prospective study of cohorts was carried out in a group of 229 healthy infants, studied from birth to 2 years of age. The infants were divided into two groups: group 1, 99 infants who were bottle-fed; and group 0, 130 who were breast-fed. All other characteristics in both groups were similar. Psychomotor development was measured between 18 and 29 months using the Bayley scales. Lower results on the Index of Mental Development were associated with bottle-fed infants, lower-middle and lower social class, elementary education of the mother, temper tantrums, and having siblings. Lower results on the Index of Motor Development were associated only with lower and lower-middle social class.