Neuroanatomical changes observed at the light microscopic level in various brain areas of four adult monkeys, who had various degrees of cerebellar ablation shortly after birth, are described in this study. Extensive neonatal hemilateral ablations of the cerebellar cortex (sparing the nuclei), which have previously been shown to leave the adult monkey with no discernible motor deficits, lead to substantial degeneration, mainly within the remaining cerebellum and the brain stem. In particular: 1) Ipsilateral to the lesion the intracerebellar nuclei and to some extent also the lateral vestibular nucleus are clearly reduced in size, whereas the contralateral cerebellum appears normal. 2) The principal olive and parts of the pontine nuclei show massive degeneration contralateral to the lesion. 3) Among the nuclei efferent to the cerebellum only the red nucleus contralateral to the lesion shows clear signs of degeneration. 4) Morphometric analysis of motor cortex and pyramidal tract reveals no systematic differences between the left and right sides, nor any other morphological indication of compensation. The morphological abnormality pattern in our monkeys is particularly similar to that described in cases of humans with olivo-pontocerebellar atrophies.