A mitochondrial DNA (mtDNA) point mutation at nucleotide pair (np) 3316 has been reported in relation to diabetes. We recently encountered a non-obese family with this type of mutation. The proband in the affected family, a 49-year-old woman who had been previously diagnosed as having an insulin-requiring non-insulin-dependent diabetes mellitus (NIDDM), was referred to our hospital for treatment of diabetic gangrene in her left foot. Her insulin secretory capacity was markedly reduced, but the insulin sensitivity evaluated by the euglycemic hyperinsulinemic clamp technique was normal. In addition, her serum lactate level was markedly increased after a 5 min ambulation, although her serum pyruvate and ketones remained within the normal range. Twenty-year-old twin sons had been treated with insulin since the age of 7, when both were diagnosed with insulin-dependent diabetes mellitus (IDDM). The proband's mother, a 68-year-old, was nondiabetic at this time. MtDNA analysis revealed a point mutation at np 3316 in all family members, which was homoplasmic for the mutation on a photograph of agarose gel electrophoresis containing ethidium bromide under ultraviolet light. This mutation seemed to be maternally transmitted in the family, and the onset of diabetes was occurring earlier and the insulin secretory capacity was declining from generation to generation, so that these findings suggest that the point mutation at np 3316 is associated with various phenotypes of diabetes.