The hypothesis 'whether subjects with attention-deficit/hyperactivity disorder (ADHD), who showed under-reactivity of the hypothalamic-pituitary-adrenal (HPA) axis to stress, would make more commission errors in attention tasks', was examined. Forty-three boys, with ADHD, who visited the psychiatric outpatient clinic, at Kangbuk Samsung Hospital, were the subjects of this study. Both pre- and post-test morning saliva samples were collected from the patients at the Korean Educational Development Institute-Wechsler Intelligence Scale for Children (KEDI-WISC), and Tests of Variables of Attention (T.O.V.A.) performed. The Standard scores of the T.O.V.A were compared between the patients with decreases, or increases, in the salivary cortisol levels after the test. Decreases, or increases in the salivary cortisol levels after the test were shown in 28 and 15 patients, respectively. The patients with decreased cortisol levels after the test tended to make more commission errors in compared with those with increased cortisol levels. The patients with the decreased cortisol levels after test had more omission errors in the first quarter of the test, and more commission errors in the second half of the test compared to those with the increased cotisol levels. Subjects who show decreased salivary cortisol levels after stress make more commission errors in attention tests. This suggests that the blunted HPA axis response to stress is related to the impulsivity in patients with ADHD.