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To study why Mexican-Americans have a threefold increase in NIDDM relative to non-Hispanic whites. The etiology of NIDDM is still controversial, with both insulin resistance and decreased insulin secretion proposed as precursors of NIDDM.We examined possible ethnic differences in fasting insulin (as a marker of insulin resistance) and change in insulin-to-change in glucose ratio (Delta I30: Delta G30) during the first 30 min after oral glucose ingestion (as a marker of abnormal insulin secretion) in 1,692 nondiabetic Mexican-Americans and 894 nondiabetic non-Hispanic whites from the San Antonio Heart Study, a population-based study of diabetes and cardiovascular disease. Fasting insulin and Delta I30: Delta G30 were evaluated as continuous variables.Mexican-Americans had increased insulin concentrations at fasting and 30, 60, and 120 min after an oral glucose load as well as an increased 0- to 30-min increment in insulin and Delta I30: Delta G30 relative to non-Hispanic whites. These results remained unchanged after adjustment for age, sex, obesity, body fat distribution, and glucose tolerance.These results suggest that increased insulin resistance rather than decreased insulin secretion is characteristic of nondiabetic Mexican-Americans, a high-risk population for NIDDM.