Hyperinsulinemia may be an early inherited marker for a defect in insulin action that subsequently results in glucose intolerance and non-insulin-dependent diabetes mellitus (NIDDM). To examine the role of hyperinsulinemia in individuals at high genetic risk for NIDDM and determine the prevalence of impaired glucose tolerance (IGT) and newly diagnosed diabetes in members of NIDDM pedigrees, we studied 310 members of 16 pedigrees ascertained for ≥2 NIDDM siblings. Nondiabetic members of all pedigrees were examined by 75-g oral glucose tolerance test with fasting and 1-h insulin levels. Participants had height and weight recorded. Spouses of pedigree members (n = 88) served as control subjects. The spouse control subjects were older and slightly more obese than the undiagnosed pedigree members. The prevalence of IGT was 14.8% in spouses and 7.7% in pedigree members, and NIDDM was present in 11.3% of spouses and 2.3% of previously undiagnosed pedigree members. However, neither spouses nor pedigree members differed significantly from published age-specific prevalence rates for IGT or newly diagnosed NIDDM. Insulin and glucose levels were examined in pedigree members with normal glucose tolerance (NGT). Fasting insulin levels were not significantly different between spouses and NGT pedigree members. However, after adjustment for age, weight (body mass index), and sex, NGT pedigree members had higher 1-h insulin levels and higher fasting and 1-h glucose levels than spouses. These differences were also evident when pedigree members with at least 1 affected (NIDDM or IGT) parent were compared with spouses with no family history of diabetes. However, when NGT pedigrees were compared by the number of parents affected, those individuals with 1 or 2 affected parents tended to have lower fasting and 1-h postglucose insulin levels and higher plasma glucose levels at 1 and 2 h than individuals with 2 unaffected parents. Although we found no evidence that younger individuals at risk for NIDDM have an increased prevalence of IGT or NIDDM by World Health Organization criteria, we did find relative hyperinsulinemia and hyperglycemia in members of pedigrees with a strong family history of NIDDM. However, within such pedigrees, offspring of affected parents tend to be hypoinsulinemic compared with their cousins with no affected parents. These findings suggest that, although hyperinsulinemia and insulin resistance characterize NIDDM pedigree members compared with control subjects, within NIDDM pedigrees, defective (β-cell function may also determine the risk of developing NIDDM.