A 26-year-old man had complaints of insidiously progressive muscle weakness of the legs, worse in the right leg than in the left. Slight to moderate degrees of asymmetrical muscular atrophy and weakness of the distal lower limb muscles, greater in the right leg than in the left, without fasciculation, were also observed. Pes equinovarus deformity of both feet was obvious. Muscle stretch reflexes were decreased in the upper limbs and absent in the lower limbs, without pathologic reflexes. Vibratory sensation was moderately decreased in the toes. The right median and tibial motor nerve conduction velocities were 19.4 and 10.5 m/sec, respectively, with a markedly prolonged distal latency. No nerve action potentials were elicited from stimulation of the right and left sural nerves. A fascicular biopsy of the right sural nerve was performed. The myelinated fibers showing segmental de- and remyelination were frequently found in teased fiber preparations. Both demyelinated and remyelinated axons and onion-bulbs were frequently observed by light and electron microscopy in the Epon-embedded sections. Based on the neurological examinations and nerve conduction studies of the family members, an elder brother, father and grandmother of the proband were found to be affected by polyneuropathy. However, the mother, an uncle, an aunt, and a cousin of the proband were normal. Therefore, we concluded that this family had HMSN type I with autosomal dominant inheritance.(ABSTRACT TRUNCATED AT 250 WORDS)