The present paper is an anatomical study of 3 cases which evolved with hypertrophy of the inferior olives: a case of zoster encephalitis, a multifocal leucoencephalopathy and a post-traumatic encephalopathy. In all three cases a relationship was demonstrated between the dentate nucleus and dento-olivary pathway - a relation of topical order in some instances - and the hypertrophic lesion of the inferior olive. Examination in the electron microscope revealed in the hypertrophic olivary cell not only dystrophic lesions but also progressive intracellular phenomena, such as maintenance of the Nissl bodies, multiplication of the mitochondria and especially marked proliferation of the neuro-filaments. This shows that, at least in some of the evolutive phases, hypertrophy of the inferior olives is based upon a real hypertrophy of the olivary cell. In addition, lesions of the glial system of the hypertrophic inferior olives were noted with the presence of osmiophil degeneration whose morphology clearly differed from that of Rosenthal's fibres.