The cerebellar cortex of progressive supranuclear palsy (PSP) cases exhibited a characteristic pathology which occurred neither in healthy aged individuals nor in cases of fully developed Alzheimer's disease. All of the 11 PSP cases studied reveal altered mossy fiber excrescences containing abnormal and hyperphosphorylated tau protein. Moreover, this abnormal material also appeared in cerebellar oligodendrocytes. Accordingly, there is not only the destruction of the cerebellar output system, which is already known, but also the involvement of the cerebellar input system.