BackgroundLow-birth-weight infants (LBW) are at an increased risk of iron deficiency that has been associated with impaired neurodevelopment. We hypothesized that iron supplementation of LBW infants improves cognitive scores and reduces behavioral problems until school age.MethodsWe randomized 285 marginally LBW (2,000-2,500 g) infants to receive 0, 1, or 2 mg/kg/day of iron supplements from 6 weeks to 6 months of age. At 7 years of age, 205 participants were assessed regarding cognition using Wechsler Intelligence Scale for Children (WISC-IV) and behavior using the parental questionnaires Child Behavior Checklist (CBCL) and Five to Fifteen (FTF).ResultsThere were no significant differences between the intervention groups in WISC-IV or FTF. However, the CBCL scores for externalizing problems were significantly different, in favor of supplemented children (P=0.045). When combining the supplemented groups, they had significantly lower scores for externalizing behavior compared with placebo (median (interquartile range): 44 [34;51] vs. 48.5 [41;56] P=0.013), and their risk ratio (95% confidence interval) for a total behavioral score above the cutoff for clinical problems was 0.31 (0.09-1.0), P=0.054.ConclusionLower scores of externalizing behavior in supplemented children support our previous findings at 3 years, and suggest that iron supplementation may have long-lasting effects on behavioral functions.