Hypertrophic changes in the inferior olivary nuclei have been occasionally described in patients with progressive supranuclear palsy (PSP). To elucidate the incidence of olivary hypertrophy, we investigated morphologically the olives and their associated pathways in 20 autopsied cases: 11 cases of PSP, 3 cases of Machado-Joseph disease and 6 cases of dentatorubropallidoluysian atrophy (DRPLA) as control diseases that usually exhibited lesions of the cerebellofugal pathway. Olivary hypertrophy was observed in 5 of 11 PSP cases, but not in the other diseases, except for 1 case of DRPLA with an old infarct in the dentate nucleus. In the olivopetal pathway, grumose degeneration of the dentate nucleus and mild neuronal loss in the red nucleus were observed in all patients with PSP and in the control subjects. Atrophy and fibrillary gliosis of the tegmentum of the pons, including that of the bilateral central tegmental tracts, were observed in all patients with PSP, more severe in the cases with hypertrophic neurons in the olives. We speculate that a lesion that involves the central tegmental tracts may play a major role in inducing hypertrophy of the olives in patients with PSP.