The Kearns-Sayre syndrome is a mitochondrial myopathy characterised by ptosis, chronic progressive external ophthalmoplegia, abnormal retinal pigmentation, and cardiac conduction defects. A unique case is reported in which there was rapid development of progressive congestive cardiac failure that required cardiac transplantation. A review of published reports of mitochondrial myopathy shows that a minority of cases (less than 20%) have cardiac involvement. This had previously been limited to abnormalities of cardiac conduction with progressive heart block. Myocardial biopsy has, however, shown ultrastructural evidence of a generalised mitochondrial disorder which hitherto has not been associated with a functional deficit.