Rett syndrome: findings suggesting axonopathy and mitochondrial abnormalities. We report the histopathologic findings of 3 sural nerve biopsies and 1 muscle biopsy from 3 patients with Rett syndrome. The 3 sural nerve biopsies demonstrated a few ultrastructural abnormalities, including the presence of many Pi-granules and mitochondrial changes in the cytoplasm of Schwann cells, occasional bands of Bungner and onion-bulb formations, and mitochondrial alterations in myelinated axons. Morphometric analysis disclosed reduction in the number of large myelinated fibers with normal densities in comparison to those of an age-matched normal control. Light microscopic examination of the biopsied muscle from a 6-year-old patient with Rett syndrome revealed the existence of many small, dark, angulated fibers with NADH-TR staining. Ultrastructural investigation of the muscle confirmed the presence of the dumbbell-shaped mitochondria. Peripheral nerve involvement and the possibility of mitochondrial abnormalities in Rett syndrome were suggested by the results. 