Comparative study of peripheral nerve biopsies and cultured skin fibroblasts from patients with late infantile and early adult-onset forms of metachromatic leukodystrophy revealed similar pathologic changes in the early stages of nerve degeneration. However, the peripheral nerves in the adult-onset cases eventually reached a more chronically demyelinated fibrotic state than did nerves in the infantile cases. Cultured skin fibroblasts from the adult-onset patients, although clearly abnormal, were able to catabolize sulfatide significantly more effectively than the cultured skin fibroblasts from late infantile patients.