Iron deficiency anemia (IDA) causes growth and developmental retardation in infants. Iron supplementation from the 4th month of age may prevent IDA, but side effects of oral iron supplementation limit its usage. The aim of this study was to investigate the effect of maternal iron supplementation on the iron status of mothers and their exclusively breast-fed infants. In a prospective, placebo-controlled, double-blinded randomized study, healthy mothers (Hb >= 11 g/dl) and their 10-20-day-old healthy term infants who were admitted to Hacettepe University for neonatal screening were enrolled. The mothers who were intending to exclusively breast-feed at least up to four months were included. Iron supplementation (n=82, 80 mg elementary iron) and placebo (n=86) were given to the mothers randomly for four months. The anthropometrical measurements of infants were recorded monthly. Of all, 69 mothers and their infants in the iron group and 63 in the placebo group completed the study. At the end of the study period, blood samples (complete blood count, serum iron, iron binding capacity and serum ferritin) were drawn from the mothers and their infants. After adjustment for baseline hemoglobin value, the mean levels of hemoglobin, serum iron and ferritin were similar in the two groups at the end of the study; however, serum iron binding capacity was significantly lower in the iron group than in the placebo group. Giving maternal iron supplementation during the first four months of the lactation period had no effect on the serum iron and ferritin levels of mothers and infants. This could be due to the relatively short duration of the follow-up period. A longer follow-up period is recommended to detect the effect of the maternal iron supplementation during lactation.