Periodic alternating nystagmus (PAN) is characterized by a periodical reversal in the direction of the nystagmus. PAN has rarely been reported in discrete cerebellar lesions. We report a patient with PAN from a focal cerebellar lesion, which developed after removal of an ependymoma in the fourth ventricle. The nystagmus decreased upon tilting the head in the direction of the nystagmus. The positional modulation of the nystagmus supports that PAN occurs from an abnormal velocity storage mechanism.