Since the identification of an A to G mutation at position 3243 of mitochondrial leucine tRNA (3243 bp mutation) in families with diabetes and deafness, the prevalence, clinical characteristics, pathophysiology of diabetes associated with 3243 bp mutation have been studied in Japan. Our study has shown that diabetes cosegregates with 3243 bp mutation in more than 30 Japanese families and that the prevalence of 3243 bp mutation in randomly selected diabetic patients (0.9%, 5 out of 550) is significantly higher than that in non-diabetic controls without family history (0.0%, 0 out of 250). Diabetes mellitus with 3243 bp mutation is often accompanied by distinct clinical features such as maternal inheritance, hearing disturbance, insulin deficiency and tendency to progression and can be regarded as a new subtype of diabetes mellitus.