We estimated the prevalence of hypertension and evaluated the degree of blood pressure control on the basis of ambulatory blood pressure monitoring in patients receiving antihypertensive medication. A total of 969 adults (mean age±SD, 59.3±12.1 years old, range: 20-79yr) among 1, 575 eligible persons (65.1%) recruited from a total adult population of 2, 789 people living in a rural region of northern Japan underwent measurement of initial screening blood pressure; ambulatory blood pressure was measured subsequently. A total of 285 subjects (66.5±9.2 years old) were taking antihypertensive medication (treated group), while 684 (56.3±12.0 years old) were not (untreated group). The WHO criteria were used to categorize screening blood pressure. Ambulatory blood pressure levels were classified as follows: hypertension, systolic blood pressure ≥144mmHg and/or diastolic blood pressure ≥85mmHg; and normotension, systolic blood pressure ≤133 and diastolic blood pressure ≤78mmHg. Of the 285 treated subjects, 49 (17.2%) were classified as hypertensive by screening measurements, while 36 (12.6%) were classified as such by ambulatory blood pressure monitoring. Only 12 (24.5%) of the former 49 subjects were also classified as hypertensive, while 20 (40.8%) were classified as normotensive by ambulatory blood pressure monitoring. Of the 684 untreated subjects, 34 (5.0%) were hypertensive by screening measurements and 43 (6.3%) were hypertensive by ambulatory blood pressure monitoring. Only 14 (41.2%) of the former 34 subjects were classified as hypertensive by ambulatory blood pressure monitoring. Of the 34 untreated subjects classified as hypertensive by screening measurements, ambulatory blood pressure monitoring showed 12 (35.3%) to be normotensive, suggesting that they were cases of "white coat" hypertension. The study first confirmed, based on community-derived data, that there are large discrepancies between screening (casual) blood pressure measurements and ambulatory blood pressure monitoring with respect to the recognition of hypertension and normotension. The determination of blood pressure levels by ambulatory blood pressure monitoring may result in a different prognosis of hypertension from that made on the basis of screening blood pressure measurements. The prognostic value of ambulatory blood pressure has to be further investigated. (Hypertens Res 1996; 19: 207-212)