Cases of unilateral vertebral artery compression associated with thoracic outlet syndrome infrequently result in symptoms and, of those that do, most involve the brain stem. Reports of transient blindness resulting from this condition are even more rare. The authors describe the case of a middle-aged woman who presented with transient blindness when she turned her head excessively to the left. She also exhibited other less severe brainstem symptoms. Arteriography demonstrated occlusion of the left vertebral artery only when her head was rotated to the left. Surgical exploration revealed entrapment of the left vertebral artery by a tight anterior scalene muscle, release of which resulted in complete resolution of her symptoms. Both neurosurgeons and radiologists need to be aware that extrinsic compression of the vertebral artery precipitated by head rotation may sometimes result in transient cortical blindness.