The accuracy of blood pressure (BP) measurement is important; systematic small errors can mislabel BP status in many persons. The objective of this study was to assess the impact of 2 types of measurement errors on the evaluation of BP in family medicine: errors associated with terminal digit preference and those associated with calibration errors of sphygmomanometers. Secondary data analyses from 2 different projects were used to derive empiric distributions of terminal digit and BP device errors. Taking into account both types of errors, the proportion of false positives (falsely high BP) and false negatives (falsely normal BP) varied between 0.82% and 5.18% of the population of consulting family physicians. In the United States, false positives and false negatives in patients' BP evaluations might lead to overtreating or undertreating 1.15 million to 7.25 million patients. Results support the need for the development of systematic interventions for quality control of BP measurements and periodic retraining for health professionals.