This study analyzed the effectiveness of weekly iron supplementation on hemoglobin concentration, nutritional status, and mental and motor development of infants at four public daycare centers in Recife, Pernambuco State, Brazil. This was a before-after intervention study conducted with weekly iron supplementation for six months in a sample of 76 infants in the 4 to 24 month age group, from February to December 2005. Mental and motor development was assessed through the Bayley Scale of Infant Development II. After supplementation, a significant increase was observed in hemoglobin concentration in the group of infants with initial hemoglobin level < or = 9.5g/dL (p = 0.001). There was also a significant increase in the mean weight-for-length z-score, but the opposite was found for length-for-age. No difference was observed in the developmental indices. We conclude that weekly iron supplementation was effective for increasing hemoglobin concentration in infants with lower initial levels, but no impact on infant development was observed.