Other investigators reported the presence of low levels of a 260-bp heteroplasmic duplication of mitochondrial DNA in patients with mitochondrial DNA deletions and their asymptomatic mothers. In this study, we were not able to detect this polymorphism in 30 patients with mitochondrial DNA deletions, but the 260-bp duplication was detected in relatively high levels (32% in muscle) in a patient with a slowly progressive mitochondrial myopathy. The duplication was also present in cultured fibroblasts (10%) and in WBC (< 1%). Mitochondrial dysfunction in this patient was evidenced in muscle by the presence of ragged-red fibers and a partial decrease in cytochrome c oxidase activity. We also detected low levels of mitochondrial DNA harboring a triplication of the 260-bp region, indicating that this polymorphism is unstable. Taken together, our results suggest that an unstable 260-bp duplication, which includes important mitochondrial DNA cis-acting regulatory sequences, may be pathogenic per se, if present at high levels.