A report is given on 29 cases of olivary hypertrophy of different origin with special reference to the quality and topistic distribution of olivary degeneration in relation to the dating and site of the primary lesion. All except one case of hypertrophic degeneration of the inferior olive resulted from injury to the dentato-olivary pathway; 13 were associated with an isolated lesion of this pathway; while in 15 cases bilateral olivary hypertrophy was secondary to multiple injury to the afferent olivary pathway. There were 12 cases of primary cerebrovascular disease, 13 cases of head injury and other lesions resulting from transtentorial herniation, and four tumours of the brain stem, one of which showed “blastomatous” olivary hypertrophy different from the usual olivary response to deafferentation. Early vacuolation of olivary neurons was seen 12 to 20 days after injury to the dentato-olivary pathway, while after longer survival olivary hypertrophy characterized by degeneration and enlargement of neurons and astroglia was associated with secondary degeneration of other parts of the dentato-olivary pathway and olivo-cerebellar tract. Correlative studies of the distribution of primary lesions and of the pattern of ensuing degeneration of inferior olives and other related structures confirmed and further defined the previously known topistic organization of the dentato-olivary pathway and the somatotopic relations between the dentate nuclei and contralateral inferior olives and the other structures involved in this neuronal circuit. The unique features of olivary hypertrophy related to transneuronal degeneration in response to deafferentation are compared with experimental data and other rare forms of olivary enlargement of blastomatous origin. Palatal myoclonus was only reported in 2 patients with old brain stem infarctions.