The frequent association of new ST-T wave changes without Q waves in the surface electrocardiogram of patients with the intermediate coronary syndrome necessitates a better understanding of the pathophysiologic significance of this finding. A previous study in patients with stable coronary artery disease indicated that the surface electrocardiogram is insensitive in detecting epicardial Q waves. This relation was evaluated in 21 patients with the intermediate syndrome, characterized by recurrent chest pain at rest associated with significant new S-T or T wave abnormalities, or both, and no new Q waves in the surface electrocardiogram at the time of open heart coronary bypass surgery. Unipolar electrograms were recorded from the epicardial surface of the left ventricle before the bypass procedure. In 19 patients, epicardial electrograms revealed initial R waves over areas of the left ventricle in which the acute S-T and T wave abnormalities were evident in the surface electrocardiogram. Two patients had epicardial Q waves (one laterally and one inferiorly). In seven patients, a transmural biopsy specimen was also obtained from the ischemic area. All showed histologically normal myocardium without evidence of early inflammatory or necrotic tissue. Of the 19 patients discharged, only one demonstrated new postoperative Q waves that had been detected by epicardial recordings before bypass. In summary, patients with the intermediate syndrome exhibiting S-T or T wave abnormalities, or both without new Q waves in the surface electrocardiogram generally do not have Q waves either in the intraoperative epicardial or postoperative surface electrocardiogram. In addition, no histopathologic abnormalities are apparent in biopsy specimens taken from the ischemic area.