THE COPIOUS literature which has arisen on hereditary ataxia has dealt for the most part with small family groups. It follows that a comprehensive picture of the disease could not be expected to emerge from the study of such families, but would be more likely to take form if data from a large family of several generations were available. The series which is the subject of this paper comprises a family derived from a common ancestor and traced in all its branches through six generations. Forty-five of the 342 members are known to have been ataxic, and records of neurologic examination are available on 22. The age of onset of ataxia is known for 32 members, including the 13 still living. The age at death is known for 27 of the 32 who have died, and the duration of ataxia, for 21. The point of particular interest, however, is the