The ultrastructural findings in the peripheral nerve of three cases of M. L. D., namely of the late infantile, the juvenile and the adult form are described. The pathological changes consist of two phenomena: segmental demyelination and lysosomal storage of sulfatides, both probably separate results of a deficiency of the enzyme Arylsulfatase A. Some differences are found in the degree of segmental demyelination and the type of lysosomal storage products. A possible relation between these differences and the supposed difference in the defect of the enzyme Arylsulfatase A, responsible for the different subtypes of the disease, is postulated.