Blood pressure measurements obtained with the Dinamap 845XT Vital Signs monitor were compared with measurements obtained with a standard mercury sphygmomanometer and a London School of Hygiene mercury sphygmomanometer in a group of 31 normotensive and hypertensive subjects. The experimental design allowed reading to be taken with all 3 devices at approximately the same time. 12 sets of readings were obtained with each device in each subject. Although inter-device differences estimated from analysis of variance were small (less than 2 mmHg after allowing for calibration differences) differences between measurements taken simultaneously with the 3 devices were often substantial. Agreement between the two mercury sphygmomanometers was better than that between either sphygmomanometer and the Dinamap. This may be a reflection of fundamental differences between auscultatory and oscillometric measurements. Differences between devices were unrelated to blood pressure level. The observed variability within subjects was similar with each device.