The economy and effectiveness of 24-hour ambulatory cassette EEG (AEEG) are increasingly recognized. We reviewed 735 AEEGs recorded between 1983 and 1989. These findings suggest that AEEG may confirm the ictal original of attacks in patients with known epilepsy, and may improve the yield of abnormality in epileptics with normal interictal EEG. Focal slowing may be seen less well on AEEG, and the yield of AEEG is low in random sampling of patients with infrequent or nonspecific symptoms.