Objective: To examine effects of iron supplementation on vigilance, attention and conceptual learning in preschool children in Greece. Design: Randomized Double-Blind Placebo Controlled trial of iron. Randomization stratified by iron status and day care center (DCC). Setting: Nine public DCCs in Athens, Greece. Subjects: In all, 49 3-4-y olds (21 anemic, 28 good iron status) with birth weight not less than 2500 g, currently healthy; benign past medical history, IQ Z1 s.d. below the age-adjusted mean, serum Pb r200 ppb (none exceeded 50 ppb), and height, weight and head circumference for age Z10th percentile. Anemia defined as: (1) pretreatment Hgb o112 g/l and TS o16% and ferritin o12 mg/L OR (2) Hgb rise of 410 g/l (T2-T0) with iron supplementation. Good iron status was defined as baseline levels of Hgb 4120 g/l and either TS 420% or serum ferritin 412 mg/l. Intervention: The intervention consisted of a 2-month supplementation of 15 mg iron (and MV) vs placebo (MV alone). Results: After iron treatment, the anemic subjects made significantly fewer errors of commission (14% higher specificity, Po0.05), exhibited 8% higher accuracy (Po0.05) and were significantly more efficient (mean difference ¼ 1.09, Po0.05) than those given placebo. These effects of iron were not found among preschoolers with good iron status. No effects of iron treatment were found on the Oddity Learning task. Conclusions: This study demonstrated that iron supplementation of iron-deficient anemic preschoolers results in an improvement in discrimination, specifically selective attention.