OBJECTIVE: To assess the precision and integrity of all aneroid and mercury sphygmomanometers regularly used by family physicians practising outside hospitals. DESIGN: Cross-sectional study. SETTING: Private medical clinics and local community health centres in Bas-Saint-Laurent, Qué. PARTICIPANTS: A total of 151 of the 166 physicians in this administrative region. MAIN OUTCOME MEASURES: Precision of the mercury sphygmomanometers was measured using the difference between a reading in the absence of pressure and level 0. Precision of the aneroid sphygmomanometers was measured using variations at pressures of 140 mm Hg and 90 mm Hg compared with those on a calibrated mercury sphygmomanometer. Integrity of sphygmomanometers, arm cuffs, and inflating bulbs was also assessed. RESULTS: In all, 258 sphygmomanometers met the inclusion criteria (111 mercury sphygmomanometers and 147 aneroid sphygmomanometers). Discrepancies of > or = 4 mm Hg were found in 15.5% of these instruments (12.6% and 17.7% of the mercury and aneroid sphygmomanometers, respectively). In 31.0% of the instruments (52.3% and 15.0% of the mercury and aneroid sphygmomanometers, respectively), one component was malfunctioning. CONCLUSION: Sphygmomanometers that measure patients' blood pressure inaccurately could result in an incorrect diagnosis of hypertension or in a normal blood pressure reading in a hypertensive patient.