To evaluate the correlation of clinical symptoms and deletion of mitochondrial DNA (mtDNA) in CPEO, we examined brain MRI, lower limb SSEP and mtDNA in 19 patients (nine men, ten women) with CPEO averaging 44.9 years of age. Of these patients, three had typical Kearns-Sayre syndrome (KSS) as defined by the presence of the invariable triad of CPEO, retinitis pigmentosa and an onset before the age of 20, as well as at least one of the followings: heart conduction block, cerebellar ataxia and elevated CSF protein. One patient was diagnosed as having probable KSS because the symptoms had begun at the age of 33. All patients with typical and probable KSS had one or more of the following common manifestations: mental retardation or dementia, hearing loss, short stature, and endocrinological disorder. All other 15 patients had ocular myopathy with limb muscle weakness. Total DNA was isolated from 19 biopsied muscles, and analyzed by the methods of Southern blot hybridization and PCR. Thirteen patients has heteroplasmy with the deleted and normal mtDNA, and six patients who had ocular myopathy did not have mtDNA deletion. The age of onset in the patients with mtDNA deletion averaged 23.0 years of age, while that without mtDNA deletion averaged 39 years of age. All KSS and two ocular myopathy patients shared the common site in mtDNA deletion, while nine with ocular myopathy showed the different sizes of deletion ranging from 2.3 to 9.5 kb in the different sites. Brain MRI was obtained from 12 of the 19 patients.(ABSTRACT TRUNCATED AT 250 WORDS)