To assess the effects of iron therapy on developmental test scores in infants with iron deficiency anemia, 68 Guatemalan babies 6 to 24 months of age, with and without mild iron deficiency anemia, were tested with the Bayley Scales of Infant Development before and after one week of oral iron treatment. The two major findings of the study were developmental deficits in the anemic group prior to treatment, and lack of rapid improvement with short-term oral iron therapy. The mean pretreatment Mental Development Index of the anemic group was significantly lower than that of nonanemic infants. The anemic group's pretreatment Psychomotor Development Index was also lower than that of the nonanemic control group. In a double-blind randomized study, six to eight days of oral iron therapy did not reverse these deficits. Consequently, the deficits of the anemic group cannot be unequivocably attributed to iron lack. However, no significant differences were found between anemic and nonanemic groups in birth histories, socioeconomic level, or general nutritional status which might otherwise explain the lower developmental test scores of the anemic babies.