Introduction The nosologic position of the leukodystrophies has long been controversial. The heredodegenerative diseases of the white matter of the central nervous system have received but scant attention in American medical literature in spite of the appearance of many case reports, review papers, and classifications in European journals during the past 40 years. In 1957, after thorough study of available pathologic material and of the relevant literature, I proposed that two basic kinds of primary diseases of the myelin sheath existed.41 The first, the myelinoclastic type, comprises the true demyelinating diseases and includes disseminated sclerosis, diffuse sclerosis of the 1912 Schilder type,47 Devic's and Balo's diseases. In this type, the myelin sheath seems to be destroyed after having been normally constituted. The second type is exemplified by the leukodystrophies. In it there appears to be a disturbance of myelin anabolism. I suggested the term "dysmyelinating diseases" to identify