In 20 patients with epilepsy, electroencephalography (EEG) slowing was quantitatively characterized during standardized hyperventilation activation (respiratory rate: 30/min, threefold elevation of total expiratory volume, duration: 4 min) and changes in cerebral blood flow and velocity in the right common carotid artery were monitored with the Doppler ultrasonic method. Thirteen age-matched normal children served as controls. The results were as follows: (1) EEG slowing in the epilepsy group was greater compared with controls. (2) There was a significant decrease in mean frequency (decrease in ci power and increase in δ power) during hyperventilation in the epilepsy group, but no significant change in the controls. (3) The decrease in cerebral blood flow (CBF) was greater in the epilepsy group at the beginning of hyperventilation, possibly related to the greater EEG slowing. (4) The percentage of CBF at the end of hyperventilation was similar in the epilepsy and control groups. The difference in EEG response to hyperventilation between the 2 groups may be due to differences in the decrease in CBF volume and the sensitivity of the change in CBF.