We found a variable defect of complex I of the mitochondrial respiratory chain, ranging in severity from 25% to 63% of control values, in muscle of patients with Huntington's disease (HD). The most severe defect was observed in the patient with the greatest expansion of CAG triplets. Muscle morphology showed myopathic changes such as moth-eaten fibers, angulated fibers, increased subsarcolemmal oxidative activities, or an increased number of enlarged mitochondria with abnormal cristae. Multiple mitochondrial DNA deletions were found by polymerase chain reaction (PCR) analysis in muscle of the patient with the most severe defect of complex I. Our data further support the involvement of energetic defects and oxidative damage in muscle of patients with HD.