OBJECTIVES: To describe age patterns of hypertension prevalence in young through middle-aged adults and to test the hypothesis that hypertension prevalence rises more rapidly with age among Blacks than Whites in the United States. DESIGN, SETTING, AND PARTICIPANTS: Using multiple logistics regression, we predicted probability of being hypertensive for participants ages 15-65 years in the National Health and Nutrition Examination Survey (NHANES) IV, 1999-2002. We estimated age-specific Black-to-White odds ratios of hypertension overall, by sex, and adjusted for body mass index (BMI) and poverty income ratio. We also followed NHANES cohorts to test whether differential age patterns of hypertension prevalence by race or gender represented cohort effects. MAIN OUTCOME MEASURE: Hypertension: systolic blood pressure > or = 140 mm Hg, diastolic blood pressure > or = 90 mm Hg, or current antihypertensive medication use. RESULTS: Black/White odds of hypertension increased from 1.71 to 3.12 between ages 15 and 65. Odds for women increased faster, from 2.11 to 4.04. By age 40, Black women had the highest hypertension rates and steepest age-gradient of race/sex groups. Adjustment for poverty income ratio did not affect results. Adjustment for BMI reduced Black women's hypertension risk somewhat but not men's. Cohort analysis confirmed a more rapid increase in hypertension prevalence among Blacks and women. CONCLUSIONS: Hypertension screening of Blacks should begin at young ages. Early diagnosis and vigilant management are critical to addressing racial and sex differences and their effect on cardiovascular disease, life expectancy, and maternal and infant health. Psychosocial stressors merit consideration as candidates for primary prevention. Addressing fundamental causes is needed. Understanding the growing age-gradient increase among US Black women is pressing.