The Dinamap automated oscillometric device (GE Medical Systems Information Technologies, Inc., Milwaukee, Wisconsin) for blood pressure measurement is widely employed in epidemiologic studies because it is easy to use and because it eliminates observer variability. In this study, the authors assessed the variability in observed blood pressures associated with use of the Dinamap monitor and estimated the contributions of various factors to that variability. In 60 volunteers (30 aged 23-35 years and 30 aged 54-82 years) from New York, New York, the authors obtained 30 simultaneous paired blood pressure measurements in both arms at 1-minute intervals, using three separate Dinamap PRO 100 devices allocated to arm and subject according to a balanced incomplete block design. Variability, defined as the between-arm difference in blood pressure measurements, was analyzed using a mixed-effects linear regression model. A total of 1,800 paired blood pressure measurements were obtained between September 2001 and June 2002. The mean ages of the two groups were 28.3 years (standard deviation, 4.0) and 71.7 years (standard deviation, 8.0). A diagnosis of hypertension was present in 53% of the older subjects and none of the younger subjects. Fifty percent of paired simultaneous blood pressure measurements obtained were in agreement within 4 mmHg for systolic blood pressure or within 3 mmHg for diastolic blood pressure. Residual variability, attributable to the intrinsic inaccuracy of the device, accounted for 64-82% of the total systolic and diastolic blood pressure variability. The majority of variability in blood pressure measurement was due to the device as used under the study conditions.