We examined the peroneus brevis (PB) muscles obtained with the sural nerves from 12 consecutively biopsied patients who were clinically and electrophysiologically ascertained to have peripheral neuropathies. Histopathological examinations revealed fiber type grouping in all of the 12 PB muscles and group atrophy associated with hypertrophic fibers in 6. These neuropathic changes were ascribable to denervation and reinnervation, being compatible with the neuropathies. Further, internal nuclei were noted in all PB muscles, and fiber splittings in 10. These myopathic changes tended to be more pronounced in the PB muscles having severe neuropathic changes and regarded as the alteration secondary to the preceding neuropathic changes. Degenerated muscle fibers, consisting of phagocytosis, basophilia and whorled fibers, were sparsely found in 8 PB muscles, while rare necrotic fibers were seen in 2. Type 1 fiber predominance was present in 7 PB muscles, in 4 of which the ratio of type 1 fiber was more than 67%. Overall, the neuropathic changes were milder in the PB muscles of CIDP patients and more marked in those showing definite axonal degeneration in the sural nerve. The severity of the neuropathic changes in the PB muscles, however, did not necessarily correlate with the strength of the leg muscles, prognoses of the diseases or the results of the nerve conduction study in the peroneal nerve. The PB muscle is liable to be affected by trauma or aging, which may modify the histopathological features of the biopsied PB muscle and therefore make the relationship between the clinical findings and the alteration of this muscle seems obscured. The available data of PB muscle have been sparse, particularly in the normal individuals, and we should be cautious in interpretation of the results of PB muscle biopsy.