Histological changes are described in the brain of a patient in whom a hemicerebellectomy had been performed 14 years before death. There is cell and fibre loss, partial or total, in nuclei known to have direct connections with the cerebellum. In some nuclei the sequel to cell loss is marked fibrillary gliosis. In other nuclei the cells have disappeared without trace. The degeneration of these tracts with a direct connection with the cerebellum has also led to degeneration of other tracts in synaptic relationship with them. This transneuronal degeneration has occurred in the corticopontine tracts and in the central tegmental tract, in the former definitely, and in the latter probably, retrogradely. It is probable that transneuronal degeneration has also occurred in other tracts. The extent of the degeneration and, in particular, the occurrence of overt transneuronal degeneration, suggest that the examination of material after a very long term of survival might be of value in experimental work concerned with mapping neuroanatomical networks.