Trigeminal evoked potentials were recorded before and 6 weeks after a percutaneous retrogasserian glycerol rhizotomy was performed in 22 patients with tic douloureux. The latency of the trigeminal evoked potential (TEP) N20 peak and the sensory threshold to electrical stimulation of the maxillary gum were measured in each patient. The N20 latency differences between normal and affected sides for each patient were reduced after glycerol injection. Sensory thresholds were significantly different between the normal and the affected sides both before and after rhizotomy. These results were interpreted to indicate that glycerol more specifically affects the damaged myelinated axons implicated in the pathogenesis of trigeminal neuralgia. Of the 22 patients, 82% were either pain-free or significantly improved after operation. Postoperative normal N20 latencies were associated with pain relief in most cases. Our preliminary experience suggests that recurrent pain is more likely to develop in those patients whose postoperative TEP remains abnormal, especially if the abnormality cannot be ascribed to a prior surgical deafferentiation procedure. In addition to providing insight into the mechanism of pain relief, TEP measurements may help to predict the likelihood of long term relief after percutaneous retrogasserian glycerol rhizotomy.