We have evaluated the accuracy of measurements provided by different types of portable automatic blood pressure monitoring devices by comparing them with intra-arterial and mercury sphygmomanometer measurements in 25 hospitalized patients. Systolic blood pressure values with portable devices that use auscultatory or oscillometric methods of measurement correlated significantly with intra-arterial values (r = 0.74 and 0.89; p less than 0.001 for both); similarly, diastolic values correlated significantly (r = 0.86 and 0.81; p less than 0.001 for both). Compared with intra-arterial measurements, there was a slight tendency for the portable devices to underestimate systolic blood pressure and overestimate diastolic blood pressure. Correlations between auscultatory or oscillometric measurements and sphygmomanometer measurements in these patients were also significant; moreover, the absolute blood pressure values obtained with the portable devices were almost identical to those with the sphymomanometer. In a further group of 12 volunteers, auscultatory and oscillometric values correlated very closely with simultaneously measured sphygmomanometer values for both systolic (r = 0.99 and 0.98) and diastolic (r = 0.96 and 0.94) blood pressures. An auscultatory device that uses continuous ECG R wave gating for Korotkoff sounds was also found to be highly accurate. Thus we have found that automated portable devices that use either auscultatory or oscillometric methods of measurement provide reliable blood pressure values.