Several studies have shown a consistent maternal effect in the transmission of Type 2 diabetes (NIDDM). The mitochondrial encephalomyopathies are a group of diseases characterized by maternal inheritance and a variety of mitochondrial DNA defects. Diabetes is a feature of some of these disorders and therefore the hypothesis arose that mitochondrial DNA mutations might play a role in patients with diabetes but no other features of neurological disease. Recent studies have confirmed that a specific point mutation in the gene encoding the mitochondrial tRNA for leucine segregates with diabetes and nerve deafness in families from the UK, Holland, France and Japan. Mitochondrial gene deletions have also been reported. Affected subjects present with progressive insulin deficiency and may fall into the broad classifications of either Type 1 (IDDM) or Type 2 diabetes (NIDDM). Future studies are aimed at searching for other mitochondrial gene defects in diabetes and attempting to explain the mechanism of hyperglycaemia by the development of phenotypic expression systems. Although an exciting development in the genetics of diabetes, currently described mitochondrial mutations do not fully explain the maternal effect in the transmission of Type 2 diabetes.