Background. Electrode-imbedded endotracheal tubes allow continuous intraoperative assessment of vocal cord function when connected to an electromyographic (EMG) response monitor. Whether this device enhances or hinders the identification and preservation of the recurrent laryngeal nerve (RLN) is unclear. Methods. The utility of continuous intraoperative neurophysiology testing (INT) of RLNs was evaluated prospectively in 100 patients undergoing 103 thyroid or parathyroid operations, involving 185 RLNs. The initial experience with 93 RLNs was compared with the subsequent 92 RLNs. Results. Overall, 97.8% of RLNs were identified intraoperatively: 1.6% visually only, 2.2% nerve stimulator only, and 94% both. There was 1 transected RLN (1.1%) in each study group. The EMG monitor could not alert the surgeon to prevent these injuries. Overall, there were 14 instances of nonfunction of visually intact RLNs (7.6%), at some point during the operation and 4 resulting in temporary paralysis (2.2%). There were 8 instances of altered RLN function (4.3%) with no altered vocal cord function postoperatively. The nerve stimulator aided dissection of the RLN in 17 instances (9.2%). There were 7 episodes (3.8%) of equipment dysfunction that hampered surgical dissection. Between study groups there was significantly increased use of the nerve stimulator to first identify the location of the RLN before visual confirmation: 4 of 93, initial group versus 25 of 92, latter group (P < .001). Conclusions. INT aids the anatomic identification of the RLN only when a positive EMG response occurs. A negative EMG response can indicate a non-nerve structure, altered function of the RLN, or equipment setup malfunction. INT cannot necessarily prevent RLN transection. © 2005 Mosby, Inc. All rights reserved.