We reviewed familial cases of chronic progressive external ophthalmoplegia (CPEO) associated with multiple mitochondrial DNA (mtDNA) deletions. A new case of familial CPEO with multiple mtDNA deletions, which were detected in the proband's skeletal muscles by Southern blotting and in all the tissues examined by using the polymerase chain reaction is also described. There was an approximate correlation between the clinical severity of the muscle involvement and the amount of mtDNA with deletions. Most of these familial CPEO cases, with multiple mtDNA deletions, exhibited an autosomal dominant mode of transmission. Abnormalities of some nucleus-driven factors, involved in the replication of mtDNA, may result in these multiple mtDNA deletions.