A revolution in chemical pathology occurred about 40 years ago with the discovery of a patient with mitochondrial dysfunction. The field of mitochondrial medicine has experienced explosive growth during the last decade. More than 50 mtDNA mutations and several nuclear gene mutations have been identified in affected patients. The recent development of animal models will continue the revolution in mitochondrial medicine by facilitating in depth studies of the molecular pathogenesis and development of novel drug and gene therapy strategies for mitochondrial dysfunction. As we enter the next millennium, we can expect mitochondrial medicine to remain a dynamic and rapidly developing field.