A family of 9 siblings included 6 boys, by 2 different fathers, all of whom demonstrated early in life profound and progressive neurological defects, together with retardation of growth and mentation and abnormal hair. Two autopsied cases showed identical pathological features, including pili torti with monilethrix and severe degenerative changes in the central nervous system. Detailed studies of these abnormal tissues, including the results of biochemical analysis (reported in part 2), has suggested that a specific genetically determined metabolic defect, leading to an aberration in fatty acid composition of the gray matter of the brain, has manifested itself as a distinct syndrome, which we have called kinky hair disease.