The aim of this prospective study was to investigate the association between breast feeding and mental and motor development at age 12 months, controlling for comprehensive measures of the child's socio-economic, maternal and environmental background, and nutritional status. A cohort of 205 infants born during May-August 2001 in a poor area in the interior of the State of Pernambuco was enrolled, of whom 191 were tested at age 12 months with the Bayley Scales of Infant Development II, and in whom breast-feeding status was measured at days 1, 10, 30, 60, 90, 120, 150, 180, 270 and 360. After adjusting for potential confounders, full breast feeding (i.e. exclusive or predominant) at 1 month was associated with a small but significant benefit in mental development (+3.0 points, P = 0.02) compared with partial or no breast feeding. No additional advantage in mental development was found with longer durations of full breast feeding. Full breast feeding at 1 month was associated with improved behaviour for two of 10 ratings tested: initiative with tasks (P = 0.003) and attention (P = 0.02). No association between breast feeding and motor development was found.