This trial aimed to investigate the effect of iron supplementation on the development of iron deficiency anemia. The study encompassed 6-month-old infants who had been exclusively breastfed in the first 4 months of life. Infants in the supplemented group were given 1 mg kg(-1) day(-1) ferrous sulfate for 6 months starting at 6 months of age. Blood samples were taken at age 12 months. A 3-day-diet was evaluated at 1 year of age. Data of 51 infants in the supplemented and 54 infants in the control group were analyzed. Mean hemoglobin values were similar in the two groups at the age of 12 months. Mean ferritin level of the supplemented group was significantly higher than that of the control. There was a significant positive correlation between dietary iron intake and hemoglobin levels. Nutrition might be more important than iron supplementation in preventing iron deficiency anemia during infancy.