Two patients with direct tandem duplications of mitochondrial DNA (mtDNA) and mitochondrial myopathy are described. The breakpoint regions between duplicated segments were amplified using the polymerase chain reaction (PCR), cloned and sequenced. The distribution of normal and abnormal genomes in different tissues was investigated using Southern hybridisation, and in different cells within the same tissue using PCR. In each case the gene for cytochrome oxidase subunit I (MTCOX1) was interrupted, creating reading frames which if transcribed and translated would result in truncated versions of this peptide. Heteroplasmy and mosaicism for the abnormal mtDNA population was apparent.