We performed a cross-sectional study in a small town in northern Japan to evaluate the distribution, reference values, and daily variation in ambulatory blood pressure. A total of 705 subjects (229 men aged 61.3±13.4 years [mean±SD] and 476 women aged 57.5±13.3 years; 41.1% of the regional adult population, n=1716), including those treated with antihypertensive drugs (n=231,66.5±9.5 years) as well as untreated subjects (n=474,55.0±13.5 years), participated in the study. Both ambulatory and screening blood pressures were measured in 659 subjects. Ambulatory blood pressure was measured with an automatic device (Colin ABPM-630). The 24-hour ambulatory blood pressure in the total population was 121.7±13.0/71.1±7.6 mm Hg (95th percentile value [95%] =146/85 mm Hg). The corresponding value in the untreated subjects was 119.4± 12.5/70.1 ±7.4 mm Hg (95%=144/83 mm Hg). The 24-hour average ambulatory blood pressure was 118.0±11.1/69.4±6.8 mm Hg (95%= 139/81 mm Hg) in subjects identified as normotensive by their screening blood pressure (n=448, 57.2±13.1 years) and 133.6±14.2/78.9±8.8 mm Hg in those identified as hypertensive by their screening blood pressure (n=73, 63.1±10.6 years). Based on the mean+SD of the 24-hour ambulatory blood pressure in the normotensive subjects by their screening blood pressure (129/76 mm Hg), the 24-hour ambulatory blood pressures in 25 (34.2%) of these 73 hypertensive subjects by screening blood pressure were below this level. Nine (2%) of 448 normotensive subjects by screening blood pressure were above the mean+2 SDs (140/83 mm Hg) of the 24-hour ambulatory blood pressure in the normotensive group by screening blood pressure. Ambulatory and screening blood pressures increased with age. The age-dependent increase in ambulatory blood pressure was less apparent in men. The 24-hour average pulse rate decreased with age. The daily variation in ambulatory blood pressure (standard deviation) increased with age, whereas that of pulse rate decreased with age. Increases in blood pressure variation were observed in nighttime and daytime blood pressure values. The differences between day versus night ambulatory blood pressures decreased with age in men but not in women. We arbitrarily cited the reference values of the 24-hour ambulatory blood pressure as mean+SD (135/79 mm Hg), mean+2 SDs (148/86 mm Hg), and 95% (146/85 mm Hg) of the ambulatory blood pressure in the total population and as mean+SD (129/76 mm Hg), mean+2 SDs (140/83 mm Hg), and 95% (139/81 mm Hg) of the ambulatory blood pressure in the normotensive subjects by screening blood pressure. Ambulatory blood pressure values above mean+2 SDs or 95% values are considered significantly elevated, and those above mean+SD values and below mean+2 SDs values are considered marginally elevated. Our results showed that the screening blood pressure values usually overestimated the prevalence of hypertension but were occasionally misleadinglv lower than the ambulatory blood pressure.