Ambulatory blood pressure monitoring (ABPM) is useful in evaluating cardiovascular risk but requires significant time. The authors examined how closely shortened time intervals correlate with the systolic blood pressure (BP) determined from a full 24-hour ABPM session in 1004 ABPM recordings. After excluding the first hour, Pearson correlations performed for the mean systolic BP of the subsequent 3-, 5-, and 7-hour periods (4, 6, and 8 hours total) with the entire, and remainder of the session, demonstrated greatest improvement in correlation when the session is increased from 4 to 6 hours. Bland-Altman analysis of the 6-hour time period revealed a mean difference of 5.41 mm Hg compared with the full session mean. The authors conclude that 6-hour ABPM can approximate the overall mean BP obtained from full 24-hour ABPM. However, shortened sessions do not characterize the influence of circadian variation on the 24-hour mean BP and may overestimate the 24-hour BP levels.