Although several studies based on psychometric tests have demonstrated an impairment of higher cortical functions in hyperthyroid and hypothyroid patients, the effect of thyroid hormones on cognitive functions has not yet been clarified. Only a few studies on auditory event-related potentials (ERP) have been reported. To investigate the cognitive effects of thyroid hormones, we studied auditory ERP in a group of 24 hyperthyroid patients, 12 hypothyroid patients, and 27 healthy control subjects. Both hyperthyroid and hypothyroid patients showed prolonged P300 latency as compared with healthy controls (P < 0.05). In addition, the P300 latency values of the hypothyroid patients were significantly longer than those of the hyperthyroid patients (P < 0,05). It is concluded that the P300 component of ERP is more vulnerable to the effect of hypothyroidism than that of hyperthyroidism. The effect of thyroid hormones on ERP is discussed.