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Ambulatory blood pressure monitoring is being used increasingly in clinical practice and hypertension research. We have noted elevated blood pressure during the initial hours of monitoring. The objective of the present study was to examine the consistency, magnitude and duration of this elevation, and to determine whether this effect causes significant differences in the mean ambulatory blood pressure monitoring values comparing the first (day 1) and second (day 2) consecutive 24-h periods of monitoring.Fifty patients who were hypertensive based on repeated clinic readings were studied prospectively. Each underwent continuous 48-h ambulatory blood pressure monitoring with a SpaceLabs 90207 monitor. The device recorded blood pressure at 15-min intervals during the daytime (0600–2159 h) and at 30-min intervals at night-time (2200–0559 h). From these readings hourly means were calculated. Repeated-measures analysis of variance was performed to compare the hourly means of days 1 and 2.Repeated-measures analysis of variance indicated that a significant difference existed for both systolic and diastolic blood pressure between day 1 and day 2. Paired Student's t-test revealed that this difference occurred during the first 2 h of monitoring. The daytime blood pressure was higher on day 1 as a result of the initial elevation of blood pressure at the onset of monitoring. The initial elevation of blood pressure was present both in white-coat hypertensives and in essential hypertensives.The first 2 h of ambulatory blood pressure monitoring are associated with elevated blood pressure both in white-coat hypertensives and in essential hypertensives. This has a minor effect on mean daytime and 24-h ambulatory blood pressures. We propose that improved ambulatory blood pressure monitoring recordings would be obtained in clinical practice, and more particularly in research applications, if 26-h ambulatory blood pressure monitoring was carried out, excluding the first 2 h from the summary analyses.