Study of the transmission of neurologic diseases from generation to generation has proved more difficult than that of infectious, vitamin-deficiency, and endocrine disorders. The gains that have been made in the latter fields have no substantial counterpart in the hereditary disorders of the nervous system. Pending further information from geneticists, chemists, and physicists, it is necessary to define and describe the scope of the problem. From the nosologic tangle that has proliferated among the hereditary degenerations of the nervous system, we are presenting observations drawn from the study of a family with a peculiar hereditary disorder having features of three separate entities. Report of Cases In May, 1946, a 6-year-old boy who exhibited signs of cerebellar ataxia and optic atrophy was brought to the office of one of us (J. A. W.). His mother said that three other members of the family were similarly afflicted. This led us to study