The static ocular torsional position of both eyes of 23 patients was measured by means of fundus photographs one day before and one week after unilateral vestibular neurectomy for the treatment of acoustic neuroma, Ménière's disease or paroxysmal vertigo. The results showed that in all patients the vestibular neurectomy caused both eyes to tort (i.e. to roll around the visual axis) toward the side of the neurectomy when measured one week after operation. The extent of this torsion was an average of 9.5 degrees one week after operation and there was no statistically significant difference in the average magnitude of the torsion in the two eyes. In 8 of these patients, additional measurements were made at intervals up to one year after operation and it was found that in these patients there is a significant reduction in torsion over time from an average of 10.2 degrees one week after operation to an average of 2.8 degrees by 16 weeks after operation. The change in torsional eye position following the neurectomy was accompanied by a change in the perceived visual orientation of a small (9.5 degrees visual angle) illuminated horizontal line at a straight ahead eye level position in an otherwise completely darkened room. One week after operation when asked to adjust the line to the perceived gravitational horizontal by rotating it in roll (i.e. around an X axis), patients who had had a right vestibular neurectomy consistently set the line so that the right side of the line (from the patient's point of view) was below the true gravitational horizontal. Similarly patients after a left neurectomy consistently set the line so that the left side of the line was below the true gravitational horizontal. There is a high correlation (r = 0.95) between the direction and magnitude of the change in torsional eye position and the direction and magnitude of the change in the perceived visual horizontal one week after operation.(ABSTRACT TRUNCATED AT 250 WORDS)