The mitochondrial DNA (mtDNA) G13513A mutation in the ND5 subunit gene has been recently reported as a common cause of some phenotypes of mitochondrial myopathy. Until now, the prevalence and characteristics of this mutation in Leigh syndrome (LS) has not been determined. We screened 84 patients with Leigh syndrome (LS) and found the mutation in six (7%) of them. The proportions of mutant mtDNA in muscles were relatively low (42–70%). The onset of symptoms for patients with this mutation was from 9 months to 5 years. It should be noted that five patients had cardiac conduction abnormalities, particularly Wolff-Parkinson-White (WPW) syndrome (three patients). This study suggests that G13513A mutation is a frequent cause of LS and that patients with this mutation may have a characteristic clinical course.