Mitochondria have a pivotal role in cell metabolism, being the major site of ATP production via oxidative phosphorylation (OXPHOS); they have a critical role in apoptotic cell death; and they also contribute to human genetics since mitochondria have a functional genome separate from that of nuclear DNA. Defects of mitochondrial metabolism are associated with a wide spectrum of disease. An Important part of this spectrum is caused by mutations of mitochondrial DNA (mtDNA). These class I OXPHOS diseases are covered in part I of this two-part review. Dysfunction of mitochondrial OXPHOS has also emerged as an important component of a range of predominantly neurodegenerative diseases in which the mitochondrial abnormality is most probably secondary. These class II OXPHOS diseases are due to mutations of genes not encoding OXPHOS subunits or are caused by exogenous or endogenous OXPHOS toxins. Class II mitochondrial diseases and the mitochondrion's role in apoptosis are covered in part II (Lancet 2000; 355: 389-94).