DNA fragmentation, the hallmark of apoptosis, has been recently investigated with contradictory results in several skeletal muscle disorders. Usingin situlabeling of nuclear DNA fragmentation, we have tested the possibility that apoptosis might occur in muscles from patients with mitochondrial respiratory chain defects and other types of metabolic myopathies. A high proportion of apoptotic myonuclei were found in all of 10 patients with mitochondrial myopathies and in one patient with multiple acyl-CoA dehydrogenase deficiency, a disease also affecting mitochondrial metabolism. These findings can be related to the intriguing link existing between apoptosis and mitochondria. It has been demonstrated that a fall of mitochondrial membrane potential constitutes a critical early event in the apoptotic process, and that mitochondrial bcl-2 protein, which protects from apoptosis, apparently functions as an endogenous permeability transition inhibitor.