Positional nystagmus may be persistent or transitory. Of the persistent forms, type I refers to nystagmus changing direction in different head positions and type II to nystagmus beating in a single direction. Both types may denote either peripheral or central abnormality. Transitory, or paroxysmal, positional nystagmus is classified as type III, and the history and physical and electronystagmographic features of its accompanying vertigo and nystagmus permit subdivision into typical and atypical forms. Guidelines are given for the differentiation of typical and atypical forms. In the great majority of cases type III positional nystagmus denotes a harmless inner ear disturbance, but the occasional serious intracranial lesion is also causative. Most instances of the latter, but not all, will fit in the atypical group. A case is reported of vermis metastasis with presenting features of postural vertigo and paroxysmal downbeat positional nystagmus.