Altered mitochondria within peripheral nerves were found in most cases of mitochondrial myopathy, in all cases of hereditary motor and sensory neuropathy with optic atrophy (HMSN VI) and in 25 cases out of a larger series of 280 unselected neuropathies studied by electron microscopy for diagnostic purposes. The mitochondrial changes differed from those seen in the corresponding skeletal muscle fibres. They comprised enlargements with an amorphous matrix and distorted cristae, hexagonal paracrystalline inclusions, sometimes longitudinally arranged in a zig-zag pattern, prominent cristae containing oblique striations and a variety of rare changes. Most mitochondrial abnormalities were found in Schwann cells. An occasional perineurial cell was also involved showing a unique paracrystalline inclusion. An increase of the number of mitochondria was noted in smooth muscle and endothelial cells of epineurial arterioles in three cases of mitochondrial encephalomyopathy (two cases with Kearns Sayre syndrome, and one with mitochondrial encephalomyopathy, lactic acidosis and stroke like episodes, i.e., "MELAS"). Neuropathy was present in all cases of mitochondrial myopathy as judged by morphometric analysis. Whether neuropathy is caused directly by mitochondrial dysfunction or by other pathogenetic mechanisms remains to be determined. Yet peripheral motor and sensory neurons with their peripheral axons are postmitotic, terminally differentiated cells which should be similarly prone to deleterious deletions of mitochondrial DNA as has been suggested as an etiologic factor for the predilection of mitochondrial diseases in muscle and brain.