To evaluate the effects of short-term iron therapy on developmental test scores of infants with varying stages of iron deficiency, 37 infants, all 15 months of age, were tested with the Bayley Scales of Infant Development before and 11 days after beginning a trial of orally administered iron therapy. They were separated into three groups according to iron status: 12 controls, with normal iron nutrition; 11 with mild anemia, i.e., hemoglobin less than 11.0 gm/dl but greater than 8.5 gm/dl; and 15 with iron deficiency without anemia, i.e., Hgb greater than or equal to 11.0 gm/dl but at least one abnormal biochemical measure of iron nutrition (transferrin saturation, free erythrocyte protoporphyrin, or serum ferritin). The Mental Development Index was significantly lower in the anemic infants before treatment, as compared with that of normal controls. Improvement with iron therapy was also significant in those with anemia and in nonanemic patients with two or more biochemical indicators of iron deficiency. The rise in Mental Development Index was associated with improvement in attention span and cooperativeness. These findings suggest that mild iron deficiency has an effect on infant behavior that is rapidly reversible with iron therapy.