Medial longitudinal fasciculus (MLF) syndrome is also called internuclear ophthalmoplegia. The syndrome is manifested by impaired adduction of the eye on the side of MLF lesion and monocular nystagmus of the contralateral abducting eye. Two cases were studied at our hospital from 1990 to 1993. The presenting symptoms were vertigo and double vision. The symptom of diplopia in one case disappeared after treatment during hospitalization, and the double vision never attacked since then . Both of the two patients were stable. Finally, we also presented the findings of electronystagmography in MLF syndrome.