We report a 37-year-old man who presented memory loss, homonymous right hemianopsia, and elevation of anti-herpes simplex antibody titer. He had an auto accident in January 1992 in that the car he was driving slipped down a 3 m slope; his car was severely damaged, however, he himself was not injured. Shortly after this accident, he went out of his house less often than before, and he noted some difficulty in his vision. He changed his glasses twice, but his vision was unchanged. In July of 1992, he had an onset of difficulty in recent memory and disorientation to time. He also noted diplopia, and difficulty in seeing objects in his right visual field. He was admitted to our hospital on August 26 of the same year. General physical examination was unremarkable. On neurologic examination, he was alert but disoriented to time and place; calculation was also impaired. Mini-mental state examination was 18/30. He had no aphasia, apraxia, or agnosia. He showed a tendency to neglect his left side. Optic fundi and visual acuity were normal; right homonymous hemianopsia was present. Ocular movement was moderately restricted to most of the directions; pupils were isocoric and reacted to light promptly. He complained of diplopia in right gaze, and monocular nystagmus was induced in his right eye upon right lateral gaze. Trigeminal nerves appeared intact. Minimum left facial weakness was present. The remaining of the cranial nerves appeared intact. His gait was wide-based and tandem gait was impossible. Muscle strength was normal as was the muscle tone. Finger to nose and heel to knee tests were done normally.(ABSTRACT TRUNCATED AT 250 WORDS)