We studied three unrelated infants and three adolescent siblings with Cockayne syndrome. The infants showed severe psychomotor retardation. Neurologic manifestations in the siblings were less severe and only slowly progressive. All patients had slowed peripheral nerve conduction. Nerve biopsies demonstrated segmental demyelination and remyelination in each case. In the infantile cases this process was severe and rapidly progressive; in the juvenile cases it was mild and chronic. Distinctive membrane-bound polymorphous inclusions were found in occasional Schwann cells.