Attention-deficit/hyperactivity disorder (ADHD) has been associated with alterations in iron metabolism, and low ferritin concentrations in peripheral blood have inconsistently been reported in clinically referred samples of children with ADHD. This study examined whether higher peripheral concentrations of ferritin, the major iron storage protein, are associated with decreased symptoms of ADHD in 2,805 children aged 10 years participating in two large population-based birth cohorts (GINIplus and LISAplus). Whether high ferritin concentrations at age 4 months predict lower ADHD symptoms at age 10 years was also investigated using a longitudinal approach in a subsample of 193 children. No indications for an association between peripheral ferritin concentrations and ADHD symptoms were found in this large population-based study. Re-evaluating iron substitution as a therapeutic measure for ADHD may be warranted.