Kearns-Sayre syndrome (KSS) is a rare cause of complete atrioventricular (AV) block in young patients.This disorder is caused by mitochondrial DNA (mtDNA) deletions, and unlike other mitochondrial diseases, involvement of the cardiac conduction system is frequent.KSS is characterized by the triad of progressive external ophthalmoplegia, pigmentary retinopathy and cardiac conduction system disturbances, with an onset before 20 years of age.We present a case of complete AV block due to this rare condition, which was diagnosed with a muscular biopsy taken at the time of pacemaker implant.