OBJECTIVE: To determine whether motor evoked potentials(MEPs)provide reliable monitoring of the motor system during resection of gliomas in or adjacent to the motor cortex or pyramidal tract. MATERIALS AND METHODS: MEP recording was performed during 64 operations in 55 patients harboring gliomas. Intraoperative MEP findings were classified into 3 groups:Group A was defined as having no significant MEP changes, Group B as having reversible MEP changes(?50% amplitude decrease or loss), and Group C as having irreversible changes. Postoperative motor function was evaluated according to the presence/absence of deterioration immediately after surgery and 1 month later, as compared to preoperative motor status RESULTS: Immediately after surgery, 13 of 39(33%)patients in Group A, 6 of 17(35%)in Group B, and 7 of 8(88%)in Group C experienced deterioration of motor function. One month after surgery, 4 of 39(10%)patients in Group A, 3 of 17(18%)in Group B, and 4 of 8(50%)showed deterioration of motor function. Both immediately(χ2=8.3, p<0.05)and 1 month(χ2=6.9, p<0.05)after surgery, MEP alterations correlated significantly with postoperative deterioration of motor function. Despite MEPs being stable throughout surgery(Group A), there were some patients with deterioration of motor function initially appearing to represent false negative monitoring. However, these deteriorations were confirmed to have been caused by secondary hemorrhage, venous return dysfunction, postoperative convulsion, or resection of the supplementary motor area. CONCLUSIONS: MEP monitoring provides reliable information on the motor system during glioma surgery. Although false negative MEP results may exist in some patients, most data were not influenced by intraoperative manipulation but rather were attributable to secondary postoperative events.