To further define a role for iron deficiency in attention-deficit hyperactivity disorder, serum ferritin was included in a battery of laboratory tests obtained in children attending a clinic for attention deficit disorders. In a total of 68 consecutive patients (ages 5-16 years; 54 male, 14 female), the range of serum ferritin levels was a low of 7.7 ng/mL and a high of 150 ng/mL. The mean (+/-S.D.) serum ferritin level in patients with attention-deficit hyperactivity disorder (39.9 +/- 40.6 ng/mL) was not different from that of control subjects. Seventy-four percent of patients had serum ferritin levels below 50 ng/mL; 44% were below 30 ng/mL; and 18% below 20 ng/mL. None had evidence of iron deficiency anemia. A comparison of the clinical characteristics of the 12 patients with the lowest serum ferritin levels (<20 ng/mL) and 12 with the highest levels (>60 ng/mL) disclosed no significant difference in severity or frequency of attention-deficit hyperactivity disorder and comorbid symptoms or response to medications. In this patient cohort, a causative role for low serum ferritin in attention-deficit hyperactivity disorder has not been confirmed.