A skeletal muscle biopsy and the nervous system of a 21-month-old male child who died of trichopoliodystrophy were studied by light and electron microscopy. The fine structural alterations in the muscle consisted of accumulation of glycogen, pronounced interfibrillar vacuolation, "hyaline degeneration," distortion of the tubular system, and disorganization of the mitochondria containing dense granules. In addition to atrophy of the brain with widespread neuronal loss and reduction of the white matter, the Purkinje cells of the cerebellum showed multiple, short, tapering cytoplasmic processes. The changes in the spinal cord consisted of neuronal loss in the Clarke's column and degeneration of the spinocerebellar tracts. Neuronal mitochondria in the postmortem material showed conglomeration and dense granules. The neuropathological changes in this disorder appear to be characteristic. The significance of the muscle changes, however, remains obscure.