To test the hypothesis that iron deficiency in infants and children is associated with behavioral alterations, 24 infants with iron deficienty anemia, ages 9 to 26 months, were randomly assigned to a treatment and control group. Bayley Scales of Infant Development were administered before treatment with intramuscular iron or placebo; the test was readministered in five to eight days. Children treated with iron showed a significant increase in their scores on the Mental Development Index averaging a mean gain of 13.6 points in a mean time of 6.8 days. The treated group was found to become more alert and responsive and demonstrated improvement in tests of gross and fine motor coordination. These findings support the hypothesis that iron deficiency in infants produced developmental alterations and that these changes are rapidly reversible with iron therapy.