Objective:To determine whether the previously reported poor reproducibility of blood pressure variability measured by ambulatory blood pressure monitoring (ABPM) is due to the uncontrolled nature of physical and mental activity during the monitoring period.Design:ABPM was performed on two separate days during which subjects performed identical activities, accompanied by the experimenter. Thus, activity and posture were controlled, both within and between subjects. Two measures of variability were used: SD and the root-mean-square of successive differences (RMSSD).Methods:Thirty-seven subjects participated. Each engaged in a series of activities, such as walking outdoors, editing and alphabetizing tasks, and eating lunch, while wearing an A & D 2420 ambulatory blood pressure monitor which took measurements at 5-min intervals. Measures of variability were computed within each session.Results:Contrary to previous reports, reproducibility was moderately high for blood pressure, with significant correlations between SD and between RMSSD for systolic and diastolic blood pressure. Heart rate reproducibility was less good.Conclusions:Lack of standardization of activities from one occasion to another is a major reason for the poor reproducibility of blood pressure variability when measured using ABPM. Even when activities are standardized, however, the reproducibility of blood pressure variability is still only moderate and may limit the ability of researchers to detect associations between ABPM variability and other measures.