We report on a case of a 48-year-old woman presenting with maternally inherited diabetes mellitus and deafness (MIDD) syndrome. Molecular genetic analysis and clinical evaluation were conducted in the patient and her 4 children to investigate the interrelation between an MIDD-associated mitochondrial DNA (mtDNA) mutation and clinical manifestations. Various symptoms and markers of MIDD, including seizures, migraines, short stature, mental retardation, and stroke-like episodes, were reviewed. Diabetes mellitus (DM) was studied by oral glucose tolerance and glucagon stimulation tests. Hearing impairment was determined by standard hearing tests and a brainstem auditory evoked potential test. The A3243G and T3271C transitional mutations of mtDNA were investigated from muscle and/or leukocytes and hair follicles. Mitochondrial-related symptoms were not found in the children, although they all harbored a heteroplasmic A3243G transition of mtDNA, as detected in screened samples. For the patient, the proportion of mutant mtDNA was highest in muscle cells followed by hair follicles and then leukocytes. Moreover, the proportion of mutant mtDNA was also higher in hair follicles than in leukocytes for asymptomatic family members. This Taiwanese MIDD family was found to have the A3243G point mutation as revealed from molecular genetic studies of leukocytes, hair follicles, and muscle tissue. However, no correlation was found between the proportion of mutant mtDNA and clinical features of any family member.