To determine reference values for ambulatory blood pressure, a population sample of 718 subjects (20 through 88 years old) was investigated. Ambulatory blood pressure was recorded over 24 h, taking measurements at 20-min intervals from 8 a.m. to 10 p.m. and at 45-min intervals from 10 p.m. to 8 a.m. Trained nurses also measured the conventional pressure at each participant's home. The ambulatory blood pressure in the 718 subjects averaged 119/71 mm Hg over 24 h, 125/76 mm Hg during the day (10 a.m.-8 p.m.), and 108/62 mm Hg at night (0 a.m.-6 a.m.). Compared with the daytime pressure, blood pressure at home was on average the same in all 718 subjects but was 3/1 mm Hg lower in 530 normotensive subjects. The 95th percentiles of the 24-h pressures were 134/ 85 mm Hg in 182 men below age 50, 145/87 mm Hg in 164 men aged ≥50 years, 127/80 mm Hg in 198 women below age 50, and 141/81 mm Hg in 174 women aged ≥50 years. In the normotensive subjects of these four strata, these percentiles were 132/82 mm Hg (n = 149), 128/80 mm Hg (n = 103), 125/79 mm Hg (n = 180), and 131/79 mm Hg (n = 98). In comparison with earlier results in a smaller sample (n = 318) from which treated patients were excluded, this analysis demonstrated consistent results in the determination of a reference frame for ambulatory monitoring. Nevertheless, the prognostic significance of such reference values needs to be established in longitudinal studies and clinical trials.