['Iron deficiency is associated with impaired dopaminergic signaling and externalizing behavior. The authors examine, whether iron stores in toddlerhood influence later response to psychostimulants.', 'Youth participating in a study monitoring the long-term safety of risperidone were included in this analysis if they had received psychostimulant monotherapy for at least 3 weeks and had a complete blood count obtained before psychostimulant treatment. Sensitivity to psychostimulants was defined based on the weight-adjusted dose during the 1st year of treatment. Regression analysis examined whether the hematological tests based on the characteristics of red blood cells were associated with sensitivity to psychostimulants.', 'A total of 29 participants (93% men; 76% Whites), primarily with ADHD (93%), comprised the current sample. The hematological tests were obtained, on average, 3 years before the initiation of psychostimulants monotherapy that occurred at 5.8 years of age and continued for a median of 0.85 years, at an average daily dose of 0.98 mg/kg (SD = 0.38) in methylphenidate equivalent. Compared with those who were poorly sensitive to psychostimulants, after adjusting for age, mean corpuscular volume was significantly higher in the highly and moderately psychostimulants sensitive groups.'] Does iron status in toddlerhood predict sensitivity to psychostimulants in children?