Previous studies of Mexican Americans have shown mean diastolic and systolic blood pressures and prevalence rates of hypertension which are either lower than or similar to those for non-Hispanic whites despite the predominance of obesity in Mexican Americans. However, those results are based on restricted samples from California and Texas. Using data from the Second National Health and Nutrition Examination Survey (1976-1980) and the Hispanic Health and Nutrition Examination Survey (1982-1984), the authors examined ethnic differences in blood pressure and hypertension. Regression analyses, stratified by sex, were used to compare mean blood pressures and rates of hypertension in Mexican Americans with those for whites and blacks. Mean diastolic and systolic blood pressures, as well as the prevalence of hypertension, were lower in Mexican Americans than in non-Hispanic whites or in blacks, with whom they shared a remarkably similar risk profile. This effect was unchanged after adjustment for age, body mass index (weight (kg)/height (cm)2 x 100), and education, indicating that blood pressure differences between Mexican Americans, whites, and blacks were not explained by the established correlates of high blood pressure. There are several possible reasons for lower blood pressure in Mexican Americans, including genetic, life-style, and cultural factors.