A rare case of monocular nystagmus is reported. A 41-year-old man complained of blurred vision on right lateral gaze for two years. When examined in our department, he showed monocular rotatory nystagmus of the right eye mainly on right lateral gaze and also on downward gaze. Neurological examinations were unremarkable. CT scan revealed a small mass lesion in the right sphenoid sinus, but the relationship betweeen the mass lesion and the abnormal eye movement was not clear. No other abnormalities were found. Though the pathogenesis remained obscure in this case, similar cases have been reported in the literature.
