Since tibial nerve somatosensory evoked potentials (SEPs) recording is influenced by hemodynamic changes and anesthetics, alterations cannot always be attributed to spinal cord ischemia, so causing false positive results. Additional recording of median nerve SEPs facilitates interpretation. From January 1988 to July 1993, 60 consecutive patients (44 men, 16 women, mean age 66 years, ranging from 26 to 83 years) underwent surgery for an aneurysm of the descending thoracic aorta using a non-heparinized left heart bypass (Biomedicus pump). In 40 patients recording of the tibial and median nerve SEPs was performed intraoperatively by stimulating both nerves alternately. In 32 patients (80%) both recordings were uneventful. In three patients (7.5%) the tibial nerve SEP temporarily disappeared due to peripheral ischemia on termination of the bypass for the creation of an open distal anastomosis. In three patients (7.5%) near loss of both tibial and median SEP recordings was caused by low blood pressure and/or anesthetics. In two patients (5%) isolated loss of the tibial nerve SEP was due to ischemia in the spinal pathway of the tibial nerve. The tibial nerve SEP signal returned to normal: in one patient after reperfusion of intercostal arteries localized within the aneurysm, in the other patient after drainage of cerebrospinal fluid (CSF). Continuous recording of both tibial and median nerve SEPs gives consistent information on spinal cord ischemia, reducing the false positive rate of the lower extremity SEP to 7.5%.