OBJECTIVE:To estimate the contribution of inadequate sphygmomanometer calibration to over- and under-detection of hypertension. DESIGN:Monte Carlo simulation of the measurement of blood pressure (BP) of a population with calibrated and uncalibrated sphygmomanometers. Simulated BP measurements included systematic sphygmomanometer error and random variability. MAIN OUTCOME MEASURES:The percentage of hypertensive adults (BP > 140/90 mmHg) not detected and the percentage of adults incorrectly classified hypertensive due to sphygmomanometer error. The percentage of the false positives and false negatives attributable to sphygmomanometer error. The number of additional visits patients need to make to obtain the same improvement in hypertension detection as is obtained by sphygmomanometer calibration. RESULTS:After three visits, uncalibrated sphygmomanometer error causes 20 and 28% of all undetected adult systolic and diastolic hypertension, respectively, and 15 and 31% of all falsely detected adult systolic and diastolic hypertension. In some groups, under-detection is worse; for example, sphygmomanometer error causes 27% of all missed systolic hypertension in 35-44-year-old females. In some age groups, over-detection is worse; for example, after three visits, sphygmomanometer error causes 63 and 50% of falsely detected systolic and diastolic hypertension in 18-24-year-old females, respectively. In-service sphygmomanometer calibration achieves the same or greater improvement in hypertension detection as an additional two visits. CONCLUSIONS:Uncalibrated sphygmomanometers are a preventable cause of clinically significant over- and under-detection of hypertension. Sphygmomanometers should be calibrated regularly by accredited organizations or technicians. Standards and guidelines governing sphygmomanometers in service should be revised. Sphygmomanometer calibration is a cost-effective way of improving hypertension detection.