This study aims to estimate the absolute number of persons with hypertension (the hypertension burden) and time trends using data from the National Health and Nutrition Examination Survey of United States resident adults who had hypertension in 1999 to 2000. This information is vitally important for health policy, medical care, and public health strategy and resource allocation. At least 65 million adults had hypertension in 1999 to 2000. The total hypertension prevalence rate was 31.3%. This value represents adults with elevated systolic or diastolic blood pressure, or using antihypertensive medications (rate of 28.4%; standard error [SE], 1.1), and adults who otherwise by medical history were told at least twice by a physician or other health professional that they had high blood pressure (rate of 2.9%; SE, 0.4). The number of adults with hypertension increased by approximately 30% for 1999 to 2000 compared with at least 50 million for 1988 to 1994. The 50 million value was based on a rate of 23.4% for adults with elevated blood pressure or using antihypertensive medications and 5.5% for adults classified as hypertensive by medical history alone (28.9% total; P<0.001). The approximately 30% increase in the total number of adults with hypertension was almost 4-times greater than the 8.3% increase in total prevalence rate. These trends were associated with increased obesity and an aging and growing population. Approximately 35 million women and 30 million men had hypertension. At least 48 million non-Hispanic white adults, approximately 9 million non-Hispanic black adults, 3 million Mexican American, and 5 million other adults had hypertension in 1999 to 2000.