An ultrastructural study of a biopsy fragment from the cerebellar cortex of a patient with ataxia-telangiectasia was performed. There was a sharp reduction in the number of Purkinje cells and an increase in the number of Golgi epithelial cells. Despite the degeneration of Purkinje cell dendrites, the volumetric density of parallel fiber varicosities and the number of synapses in the molecular layer were only slightly reduced. No qualitative nor quantitative abnormalities were noted in the granular layer. An analysis is made of the severe changes in cerebellar circuitry of this patient and their probable functional implications.