We studied six unrelated adults with mitochondrial myopathy due to the m.3243A>G MTTL1 mutation in skeletal muscle (50–84% heteroplasmy, four female, mean age 50 years, range 44–57), identified from local clinics and the UK MRC Mitochondrial Disease Cohort. The gene discussed is MT-TL1; the disease is Mitochondrial myopathy.