BACKGROUND Iron deficiency causes abnormal dopaminergic neurotransmission and may contribute to the physiopathology of attention-deficit/hyperactivity disorder (ADHD). OBJECTIVE To evaluate iron deficiency in children with ADHD vs iron deficiency in an age- and sex-matched control group. DESIGN Controlled group comparison study. SETTING Child and Adolescent Psychopathology Department in European Pediatric Hospital, Paris, France. PATIENTS Fifty-three children with ADHD aged 4 to 14 years (mean +/- SD, 9.2 +/- 2.2 years) and 27 controls (mean +/- SD, 9.5 +/- 2.8 years). MAIN OUTCOME MEASURES Serum ferritin levels evaluating iron stores and Conners' Parent Rating Scale scores measuring severity of ADHD symptoms have been obtained. RESULTS The mean serum ferritin levels were lower in the children with ADHD (mean +/- SD, 23 +/- 13 ng/mL) than in the controls (mean +/- SD, 44 +/- 22 ng/mL; P < .001). Serum ferritin levels were abnormal (<30 ng/mL) in 84% of children with ADHD and 18% of controls (P < .001). In addition, low serum ferritin levels were correlated with more severe general ADHD symptoms measured with Conners' Parent Rating Scale (Pearson correlation coefficient, r = -0.34; P < .02) and greater cognitive deficits (r = -0.38; P < .01). CONCLUSIONS These results suggest that low iron stores contribute to ADHD and that ADHD children may benefit from iron supplementation.