Fifteen cases of mitochondrial myopathy, three cases of hereditary motor and sensory neuropathy (HMSN) VI, and 280 cases of neuropathies of different etiologies were examined by electron microscopy for the presence of mitochondrial abnormalities in the sural nerve. Altered mitochondrial were found in most cases of mitochondrial myopathy, in all cases of HMSN VI, and in 25 cases out of the series of unselected neuropathies. The mitochondrial changes comprised enlargement with an amorphous matrix and distorted cristae, with hexagonal paracrystalline inclusions, and with prominent cristae containing oblique striations, and a variety of rare changes. Most mitochondrial abnormalities were found in Schwann cells. An increase of the number of mitochondria was noted in smooth muscle and endothelial cells of epineurial arterioles of two cases with mitochondrial encephalomyopathy. Neuropathy was present in all cases of mitochondrial myopathy according to morphometrical analysis. Whether neuropathy is caused directly by mitochondrial dysfunction or by other pathogenetic mechanisms remains to be determined.