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To evaluate associations between iron supplementation and iron deficiency in infancy and internalizing, externalizing, and social problems in adolescence.The study is a follow-up of infants as adolescents from working-class communities around Santiago, Chile who participated in a preventive trial of iron supplementation at 6 months of age. Inclusionary criteria included birth weight ≥3.0 kg, healthy singleton term birth, vaginal delivery, and a stable caregiver. Iron status was assessed at 12 and 18 months of age. At 11–17 years of age, internalizing, externalizing, and social problems were reported by 1018 adolescents with the Youth Self Report and by parents with the Child Behavior Checklist.Adolescents who received iron supplementation in infancy had greater self-reported attention-deficit/hyperactivity disorder but lower parent-reported conduct disorder symptoms than those who did not (Ps < .05). Iron deficiency with or without anemia at 12 or 18 months of age predicted greater adolescent behavior problems compared with iron sufficiency: more adolescent-reported anxiety and social problems, and parent-reported social, post-traumatic stress disorder, attention-deficit/hyperactivity disorder, oppositional defiant, conduct, aggression, and rule breaking problems (Ps < .05). The threshold was iron deficiency with or without anemia for each of these outcomes.Iron deficiency with or without anemia in infancy was associated with increased internalizing, externalizing, and social problems in adolescence.