The Dinamap 845XT automatic blood pressure monitor, Dinamap 8100 (an update model), and two mercury sphygmomanometers were compared in 417 school-aged children examined in the spring of 1992 as part of the Bogalusa Heart Study. This study was conducted in the nearby community of Franklinton, Louisiana, to verify data obtained as part of a cross-sectional survey (1987 to 1988) of school-aged children in Bogalusa. Systolic blood pressure levels were on the average 3 mm Hg higher on the Dinamap instruments than on the sphygmomanometers. Mean levels of diastolic blood pressure using either Dinamap instrument were slightly higher until eight years of age and then were considerably lower than mercury sphygmomanometer fourth phase readings. Diastolic blood pressure levels on the Dinamap 8100 were 4 mm Hg lower than on the Dinamap 845XT. Height was identified as the predominant predictor variable of differences in diastolic blood pressure between the mercury sphygmomanometer and either Dinamap instrument. A 10% random sample of children was reexamined each screening day in the cross-sectional survey to estimate measurement errors. The diastolic readings of the Dinamap 845XT had a lower intraclass correlation (0.68) compared to the mercury sphygmomanometers (0.83 fourth phase and 0.76 fifth phase). The Dinamap offers the ease of measuring systolic blood pressure although the diastolic blood pressure appears to be biased and especially low, particularly on the new 8100 model.