To evaluate the autonomic nervous system of patients with breath-holding spells after iron treatment, we attempted to determine whether a dysregulation of the autonomic nervous system reflexes exists in children with severe cyanotic breathholding spells. An electrocardiogram for each subject was recorded for 24 hours in the subject's home and parasympathetic activity was investigated by the fast Fourier transform method. Hematologic data and clinical symptoms of all three patients treated with iron improved and attacks of severe breath-holding spells disappeared. After iron treatment was started, the heart rate variability increased during sleep. It appears that supplementation of iron is effective in improving the dysregulation of autonomic nervous system reflexes.