Recently, subtypes of typical NIDDM were suggested based on missense mutations of mitochondrial DNA [tRNALeu(UUR)] and the glucagon receptor gene (Gly40Ser). Together these mutations might explain NIDDM in 5--8% of patients. To test the hypothesis that these mutations play an important role in a Northern European population with a strong family history of diabetes, we screened members of 45 families selected for having two or more diabetic siblings and 62 additional unrelated diabetic individuals for both mutations. We also examined 74 nondiabetic control subjects. Mitochondrial DNA mutations were not detected despite our ability to detect as little as 3% heteroplasmy in a sample from an individual known to carry the mutation. Likewise, the glucagon receptor Gly40Ser mutation was present in a single diabetic patient who on subsequent investigation was of Italian descent. Thus, neither subtype of NIDDM is present in the Utah diabetic population, which is reflective of other Northern European populations.