SUMMARY We determined the respiration, respiratory control, and Pi:O ratios with different substrates in mitochondria isolated from five cases of human neuromuscular disorders (two cases of central core disease, two cases of neuropathy of Dejerine-Sottas, and one case of KugelbergWelander's disease) and compared them with normal human muscle. In all the myopathies studied, a severe derangement of the respiratory control with variable derangement of oxidative phosphorylation was found. This supports the idea that a group of neuromyopathies shares the same biochemical lesion as the so-called mitochondrial myopathies, forming with them a group of myopathies which may be related through a similar biochemical lesion of varying degree. Alternatively, disturbance of mitochondrial functions in a number of myopathies could be considered as a non-specific finding.Despite the frequent occurrence of morphological changes in mitochondria in many types of human neuromuscular disorders (Gruner, 1963;Aleu and Afifi, 1964;Gonatas, Perez, Shy, and Evangelista, 1965;Norris and Panner, 1966;Gonatas, 1967;Engel and Dale, 1968;Chou, 1969), several myopathies have been described in which abnormalities of this organelle were -considered to be of major significance (Luft,