The mitochondrial respiratory chain function and the occurrence of mitochondrial respiratory chain dysfunction were determined in various neuromuscular diseases. The mitochondrial complexes I–V and citrate synthase in the skeletal muscle taken from 75 orthopaedic surgical patients excluding neuromuscular diseases (control subjects) and 26 patients with various neuromuscular diseases (7 patients with Duchenne muscular dystrophy, 3 patients with spinal muscular atrophy, 6 patients with mitochondrial diseases, 7 patients with type II fibre atrophy and 3 patients with neuropathy) were assayed. Of 26 patients, results of analysis of 3 patients (1 Duchenne muscular dystrophy, 1 spinal muscular atrophy and 1 type II fibre atrophy) were excluded because the citrate synthase activities in their muscle homogenate were less than third percentile of the normal controls. As compared to the control subjects by using Student’s t-test, all studied groups of patients had significantly lower activities of more than one or two mitochondrial complexes (p<0.05). However, a significantly higher activity of mitochondrial complex I was observed in patients with mitochondrial diseases (p<0.05). These findings will require further study to elucidate the pathogenesis and role of secondary mitochondrial respiratory chain dysfunction in such neuromuscular diseases.