24-hour ambulatory blood pressure (BP) was monitored by non-invasive methods in 42 offspring of hypertensive families (age [mean (SD1)] 40(7) years) and 38 offspring of normotensive families (age 33(6) years). The family history was defined according to parental BP data from two surveys conducted 27 years apart. Casual BP was 137(17)/84(12) mmHg in offspring of hypertensive families and 117(9)/69(6) mmHg in offspring of normotensive families (difference: p < 0.001). Average 24-h BP was 123(10)/74(6) mmHg and 113(8)/65(5) mmHg, respectively (difference: p < 0.001). The systolic and diastolic BP difference of approximately 10 mmHg was observed between the groups throughout the monitoring period. Hypertension--defined according to a recent meta-analysis as an average 24-h BP > or = 139/87 mmHg--was found in 6 offspring of hypertensive families and in no offspring of normotensive families (p < 0.05). The 24-h systolic and diastolic BP load--the percentage of readings above 140/90 mmHg (day-time) and 120/80 mmHg (night-time)--was higher in offspring of hypertensive than normotensive families (27%/17% vs. 7%4%; p < 0.001). After adjustment for intrafamilial covariation, age, and other possibly confounding variables, the differences between the groups remained. The present findings suggest that BP in subjects with a family history of hypertension is elevated on a permanent basis, and not only when it is measured in the doctor's office.