This population study included 399 subjects, of whom 370 (93%) showed a significant diurnal blood pressure (BP) rhythm. The nocturnal BP fall was normally distributed and averaged 16 ± 9 mm Hg systolic and 14 ± 7 mm Hg diastolic (mean ± SD). The amplitude of the diurnal BP curve followed a positively skewed distribution, with a mean of 16 ± 5 mm Hg for systolic BP and 14 ± 4 mm Hg for diastolic BP. The daily BP maximum occurred at 15:54 ± 4:47 for systolic BP and at 15:11 ± 4:20 for diastolic BP. Thirty-four subjects were reexamined after a median interval of 350 days. The test for the presence of a significant diurnal rhythm was discordant in only two subjects. Repeatability (twice the standard deviation of the differences between paired recordings expressed as a percentage of the mean) varied from 11 to 25% for the 24 h, daytime, and overnight BP, and from 76 to 138% for the parameters describing the diurnal BP rhythm. In nine subjects with an initial night/day ratio of mean BP < 0.78, the nighttime BP was significantly increased at the repeat examination, whereas the opposite tendency was observed in nine subjects with an initial ratio > 0.87. In conclusion, the distribution of the nocturnal BP fall is unimodal. The re-producibility of the ambulatory BP is satisfactory for the level of BP and for the presence of a diurnal BP rhythm, but not for the parameters of the diurnal BP curve. Thus, one 24 h recording is insufficient to fully characterize an individual’s diurnal BP profile. Am J Hypertens 1992;5:386-392