We performed a randomized, single-visit, crossover study to compare the precision of a digital monitor and an aneroid monitor relative to a desktop mercury sphygmomanometer (DMS). Three blood pressure measurements per patient, one per device, were taken over 25 minutes. Of 99 patients, 95 had complete blood pressure data. Systolic blood pressures measured with the digital monitor were significantly different from those with the DMS (p<0.01). Thirty-four percent of systolic and 48% of diastolic pressures measured with the digital monitor were within +/- 5 mm Hg of the DMS. The aneroid monitor performed significantly better than the digital monitor, with 54% of systolic (p<0.01) and 58% of diastolic blood pressures within +/- 5 mm Hg of the DMS standard. Health care providers are encouraged to supervise patients' initial use of a home blood pressure monitor and calibrate readings against a standard mercury sphygmomanometer.