SUMMARYPeripheral nerve and muscle of patients with hemiatrophy were biopsied and examined by means of light and electron microscope.The unmyelinated nerve fibers decreased in number and remaining fibers were atrophic with irregular shape and high electron density, while myelinated nerve fibers slightly decreased in number. The Schwann cells enveloping nerve fibers were also atrophic with high electron density.The muscles contained numerous glycogen granules in the myofilaments and between the myofilaments. The mitochondria in the muscles showed atrophy with high electron density. Some parts of severe atrophy showed honeycomb‐like structure forming large vacuoles in the muscle fibers.From the findings mentioned above, it is speculated that primary change was nerve fibers, especially unmyelinated nerve fiber