Hyperinsulinemia has been demonstrated in Hispanics with normal glucose tolerance and in other populations at higher risk for non-insulin-dependent diabetes mellitus (NIDDM). We compared fasting and glucose-stimulated insulin and C-peptide levels in a community-based sample of 464 Hispanic and 676 non-Hispanic white adult residents of the San Luis Valley of Colorado. All subjects had normal glucose tolerance as confirmed by oral glucose tolerance testing interpreted with World Health Organization criteria. Mean fasting and 1- and 2-h post-glucose load insulin levels were significantly higher in Hispanics versus non-Hispanic whites (fasting 0.08 vs. 0.07 nM, P = 0.0026; 1 h 0.52 vs. 0.47 nM, P = 0.0129; 2 h 0.36 vs. 0.27 nM, P less than 0.0001), even after adjustment for age, sex, body mass index, waist-hip ratio, family history of diabetes mellitus, concurrent plasma glucose level, and fasting insulin level. Mean fasting and 1- and 2-h glucose-stimulated C-peptide levels in Hispanics also significantly exceeded those in non-Hispanic whites (fasting 0.58 vs. 0.54 nM, P = 0.0119; 1 h 2.72 vs. 2.46 nM, P less than 0.0001; 2 h 2.25 vs. 1.97 nM, P less than 0.0001). The C-peptide-insulin molar ratio was greater in non-Hispanic whites than Hispanics at all times measured. These findings confirm that Hispanics with normal glucose tolerance are hyperinsulinemic and that increased insulin secretion is at least partly responsible for this phenomenon. The lower levels of C-peptide compared with insulin in Hispanics suggest that the hyperinsulinemia seen in this ethnic group may be due in part to decreased hepatic insulin extraction.