Forty-three cases of peripheral neuropathy (PN) have been reported in the literature with a proven mitochondria (mt) DNA mutation, and 21 had a peripheral nerve biopsy (PNB). We studied 8 patients, 1 of whom had severe sensory PN, 3 mild PN, and 4 subclinical PN. Nerve biopsy was performed in every case; all patients showed axonal degeneration and 4 showed features of primary myelin damage. In addition, there were 2 crystalline-like inclusions in the Schwann cell cytoplasm of a patient with MERRF, and 1 in a patient with multiple deletions on the mtDNA. There are 11 cases of PNB in the literature with axonal lesions, 5 with demyelination, and 4 with mixed lesions. One PNB was not modified. A few crystalline-like inclusions were seen in 1 case of MERRF. Such inclusions were first reported in the Schwann cell cytoplasm of unmyelinated fibers in a patient with Refsum disease and were considered to be modified mitochondria. However, their mitochondrial origin remains debatable.