Mitochondrial DNA is more susceptible than nuclear DNA to mutations. Mitochondrial mutations have been associated with a range of disorders, some of which can be inherited maternally as well as by mendelian patterns. The oxidative phosphorylation diseases are a group of such disorders characterised by a complex phenotype; the Kearns-Sayre syndrome, for example, can include cardiac abnormalities, diabetes mellitus, cerebellar ataxia, and deafness. An understanding of the genetic and biochemical basis of these disorders will help in the adoption of a systematic approach to their diagnosis and to patient management.