Kearns-Sayre syndrome is a neuromuscular disease characterised by an onset before the age of 20 years, ophthalmoplegia, ptosis and pigmentary retinitis. More than 200 cases have been published. The prevalence is estimated between 1 and 3/100 000. The disease often starts with the hallmark ocular symptoms, followed by the progressive occurrence of several other signs, depending on the tissue distribution of the molecular anomaly. The most frequently associated symptoms include deafness, heart involvement (cardiomyopathy, cardiac conduction defect), cerebral involvement (ataxia, high cerebrospinal fluid protein content, intellectual deficit), skeletal muscle myopathy, intestinal disorders, hormonal deficit (hypoparathyroidism, diabetes), and renal failure. The disease progresses slowly, with new symptoms appearing and previous symptoms slowly worsening. A very few cases of Pearson syndrome (see this term) have progressed into Kearns-Sayre syndrome. Kearns-Sayre syndrome is caused by deletions of large portions of mitochondrial DNA. Deletions are heteroplasmic, i.e., a single cell can harbour both deleted and normal DNA molecules. Symptoms only appear if the proportion of abnormal DNA is high. The threshold depends on the organ; about 60% for the skeletal striated muscle. Most cases of Kearns-Sayre syndrome are sporadic. In fact, deletions of mitochondrial DNA are only exceptionally transmitted from one generation to the next. The diagnosis is suggested by the clinical picture and by the presence of typical morphological alterations in the skeletal muscle (fibres presenting with mitochondrial proliferation or 'Ragged Red Fibres' and cytochrome c oxydase deficient fibres). It can be confirmed by the detection of high proportion of deleted mitochondrial DNA in a clinically or morphologically affected tissue (usually in the skeletal muscle). Differential diagnosis includes forms with an overlapping clinical picture, such as Pearson syndrome or chronic ophthalmoplegia. Treatment of the various symptoms is supportive. The prognosis essentially depends on the number of organs involved. The disease progresses slowly over decades.