The prevalence of non-insulin dependent diabetes mellitus (NIDDM) is higher in Mexican Americans (MAs) than in non-Hispanic whites (NHWs) even after adjustment for the former's greater overall and more centralized adiposity. We previously have shown that MAs are more hyperinsulinemic than NHWs even after adjustment for body mass index (BMI), waist-to-hip ratio (WHR), subscapular-to-triceps skinfold ratio, and glucose tolerance. We now confirm these findings in much larger population (n = 1,182). Since hepatic extraction of insulin may be decreased with upper body adiposity, we also measured fasting and 2-hour C-peptides in a subset of subjects (n = 70). C-peptide concentrations were higher in MAs than in NHWs indicating that MAs have higher insulin secretion. We examined whether the increased insulin concentrations in MAs may be associated with a bimodal distribution of insulin concentrations in which the second or higher mode might represent pre-diabetics. No evidence of bimodality, however, was apparent. The effect of overall adiposity (BMI) and upper body adiposity was similar in both Mexican Americans (a high risk population for NIDDM) and non-Hispanic whites (a low risk population for NIDDM). These data indicate that, like other populations at high risk for NIDDM such as Pima Indians in the U.S. Southwest and Micronesians in the South Pacific, Mexican Americans have more hyperinsulinemia than can be accounted for by their adiposity and body fat distribution. Prolonged insulin secretion (secondary to increased insulin resistance), is one possible explanation for this phenomenon which could lead to islet cells decomposition and eventual clinical diabetes.