THE MEXICAN-AMERICAN POPULATION in the United States has generally elevated frequencies of several chronic conditions, including non-insulin-dependent diabetes mellitus (NIDDM), gallbladder disease, and obesity. Prevalence of cardiovascular disease and hypertension is less clear. To document prevalence and risk factors of hypertension in this population, we measured blood pressure in 1004 randomly selected Mexican Americans in Starr County, Texas, ages 15 to 74. We defined hypertension as systolic blood pressure greater than or equal to 140 mmHg or diastolic pressure greater than or equal to 90 mmHg or current (within the last 48 hours) use of antihypertensive medications. Prevalences by age and gender are elevated in this population group compared with those in the general population. In addition to age and gender, body mass and diabetes status were also predictors of hypertension. Comparison of the Starr County results with those reported from the Third National Health and Nutrition Examination Survey (NHANES III) sampling of Mexican Americans indicates a slight increase in frequency of hypertension in Starr County, while comparison with results from San Antonio Mexican Americans indicates a marked increase in frequency in Starr County. These differences are not simple functions of measurement protocols, but are likely to be caused by differences in population structure, employment and socioeconomic status, education, and other such factors.