Clinical, radiological and electrophysiological data in two patients with 'Miller Fisher syndrome' (MFS), characterized by ataxia, ophthalmoplegia and areflexia are reported. Of the many tests performed only the electrophysiological findings provided any clue to the site of the lesions and the structure involved, showing a significant pattern of brainstem dysfunction. The authors discuss the role of central and peripheral lesion theory in the light of these data and stress that electrophysiological findings lend considerable weight to the former and support the hypothesis that MFS should be recognize as a distinct clinical entity.