To determine ambulatory and home blood pressure means and distributions in relation to clinic blood pressure in a general population.We obtained a random sample of 2400 subjects stratified by sex and 10 year age groups to be representative of residents aged 25-64 years of the city of Monza. Participation rate was 69% (1651 subjects). Blood pressure measurements consisted of clinic blood pressure (average of three measurements, sphygmomanometry), home blood pressure (average of morning and evening measurements, semiautomatic device) and ambulatory blood pressure (automatic readings at 20 min intervals, Spacelabs 90207). Clinic blood pressure was obtained both before and after home and ambulatory blood pressures. Data analysis did not include 213 subjects receiving antihypertensive drug treatment and was therefore limited to 1438 participants.In the 1438 subjects, clinic, home and ambulatory blood pressure showed a normal-like distribution, with a taller peak and a narrower base for ambulatory than for home and clinic values. Clinic, home and ambulatory blood pressures were significantly related to each other (P always < 0.001). The means of the two clinic blood pressures obtained on consecutive days were superimposable (127.4 +/- 17.0/82.3 +/- 9.8 and 128.2 +/- 16.5/81.9 +/- 9.9 mmHg) and both were markedly higher than home and 24 h average blood pressures (8.2 mmHg), which were similar to one another. The differences between clinic and home or 24 h average blood pressure were similar in both sexes but increased with increasing age and clinic blood pressure values. The influence of clinic blood pressure values on the clinic-ambulatory or clinic-home blood pressure differences was more important than age. Although higher than the 24 h average value, daytime average blood pressure was also lower than clinic blood pressure. Night-time blood pressure was markedly lower than the daytime value in both sexes and at all ages.Data from a large and unbiased sample of a general population show that home and 24 h or daytime average blood pressures are much lower than clinic blood pressure. The relatively close correlation between blood pressure values measured with the different methods used has allowed calculation of home and ambulatory blood pressure values corresponding to the accepted upper limit of normality of clinic blood pressure (140/90 mmHg). The upper limit of normality for the population was for both home and ambulatory blood pressures in the range 120-130 and 75-81 mmHg for systolic and diastolic values, respectively, with slight differences depending on sex and age. Taking 140/90 mmHg as the upper normal limit of the population is therefore an error that leads to individuals whose home or ambulatory blood pressures are high being considered as normotensive.