Twenty patients with mitochondrial myopathy were investigated for the presence of peripheral neuropathy. There were clinical features of a mild sensorimotor neuropathy in five patients (25%) and nerve conduction studies were abnormal in 10 patients (50%). Electrophysiological studies of the whole group showed significant impairment of motor and sensory conduction compared with controls. Sural nerve biopsy and morphometric studies were performed on four patients with clinical neuropathy. There was a reduction in density of myelinated fibres and electron microscopic features of axonal degeneration affecting myelinated and unmyelinated fibres were in evidence. Abnormal mitochondria containing paracrystalline inclusions were seen in the Schwann cell cytoplasm of two nerves.