Spinal cord evoked potentials elicited by direct stimulation of the spinal cord were monitored in 21 patients during thoracic or thoraco-abdominal aortic aneurysm surgery. Flexible catheter-type electrodes were used for both stimulating and recording. The basic pattern of the spinal cord evoked potential consisted of an initial spike and a subsequent polyphasic component. The earliest and most frequent alterations after cross-clamping of the aorta were changes in the configuration or amplitude of the polyphasic component. In 13 patients who exhibited no change except minor alterations of the polyphasic component during the initial test clamping for 15 or 20 min, subsequent graft replacements were safely performed without reimplantation of intercostal vessels. In 2 patients who had sudden cardiac arrests, the evoked potential completely disappeared. The polyphasic component disappeared first, followed by the initial spike. Another patient developed acute loss of the potential after the aneurysm was incised, presumably due to distal aortic hypoperfusion. In this case, prolonged distal hypotension resulted in flaccid paraplegia. Intraoperative monitoring of the spinal cord evoked potential is a useful method for the early detection of spinal cord ischemia during surgery requiring aortic occlusion.