A double-blind clinical trial was conducted in Indonesia to assess effects of iron supplementation on performance of iron-depleted and iron-deficient anemic children in discrimination and oddity learning tasks. Half these children received elemental Fe for 8 wk; the others received a placebo. There were significant changes from pre- to postintervention evaluations in ferritin, transferrin saturation, free erythrocyte protoporphyrin, and hemoglobin among the anemic and iron-depleted children; no changes were observed among the placebos or any of the iron-replete children. The magnitude of hematological changes in anemic children treated with iron was small; yet, after treatment the children's mean ferritin, transferrin saturation, and hemoglobin values were above the cutoff points used for the definition of iron-deficiency anemia (IDA). Pre- and posttreatment psychological test data show that IDA produces alterations in cognitive processes related to visual attention and concept acquisition, alterations reversed with iron treatment.