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Increasingly, patients measure and record their home blood pressure. However, the accuracy with which they report their readings to their physicians is largely unknown. The authors assessed the accuracy and quality of self-reported home blood pressure values in an ambulatory managed care population. Forty-eight hypertensive outpatients were randomly allocated to either receive information about the storage capabilities of a home blood pressure measuring device or not to receive such information. All patients were asked to record the measurement results in a logbook twice daily over a 7-day period. The main outcome measure was the difference in the number of fictional or manipulated reports per group and the difference in missing values. The combined parameter “manipulated or fictional registrations” occurred significantly less frequently in the informed group than in the noninformed group. (10/728 vs. 29/616; relative risk, 0.292; 95% confidence interval, 0.15–0.57; Pearson X2=13.15; p<0.0001). Informed patients had fewer missing registrations than the noninformed (13/728 vs. 41/616 measurements; relative risk, 0.27; 95% confidence interval, 0.15–0.47; Pearson X2=20.5; p<0.0001). The mean of the fictional data did not differ systematically from the mean of the correctly reported individual blood pressure values. There was no trend to over- or underestimate blood pressure values in the noninformed group. With this study design, it was possible to identify manipulation of home blood pressure values for the first time. Accuracy and interpretation of home blood pressure measurement may be increased by using devices with a memory function.