Trigeminal neuralgia is unique to humans. The most common cause seems to be an injury to the myelin of the trigeminal nerve root entry zone as it extends for several millimeters lateral to the pons. Jannetta has developed an elegant retromastoid microsurgical approach to this region. He has identified a compression-distortion phenomenon of this nerve root entry zone, usually from an anomalous position of the superior cerebella artery. Trigeminal neuralgia can also occur in association with multiple sclerosis, when the plaque lies in this same location. The historical evidence for this explanation is reinforced by the electron microscopic demonstration of the plaque in this region in a patient with multiple sclerosis who was suffering from tic douloureux. (Neurosurgery, 5: 711--717, 1979).