The authors previously reported that intrauterine exposure to background concentrations of p,p'-dichlorodiphenyltrichloroethane (DDT) reduces cognitive performance among preschoolers. Breastfeeding has been associated with both increased exposure to certain pollutants during infancy and better performance on cognitive tests. Thus, the authors examined the role of breastfeeding in cognitive function among preschoolers, taking prenatal DDT exposure into account. Two birth cohorts in Spain (Ribera d'Ebre and Menorca) were recruited between 1997 and 1999 (n = 391). Infants were assessed at age 4 years using the McCarthy Scales of Children's Abilities. Levels of organochlorine compounds were measured in umbilical cord serum. Information on type and duration of breastfeeding was obtained by questionnaire when the children were 1 year of age. Children who were breastfed for more than 20 weeks had better cognitive performance regardless of their in utero exposure to DDT. A linear dose response between breastfeeding and cognition was observed in all DDT groups (for children highly exposed to DDT, adjusted beta = 0.30 (standard error, 0.12) per week breastfed). Despite the possibility of harm from environmental contaminants in breast milk, breastfeeding for long periods should still be recommended as the best infant feeding method.