A new continuous, non-invasive arterial blood pressure monitoring system based on arterial tonometry was currently developed. A sensor of this system consists of an array of 15 piezoresistive pressure transducers. An automatic sensor positioning system ensure that at least one of the 15 transducers is positioned on the center of the pneumatically flattened, but not occluded, portion of the artery. Accuracy, reliability and clinical acceptance of this system were evaluated by comparing tonometric blood pressure (TBP) measurements with intra-arterial blood pressure (IBP) measurements in 70 normotensive anesthetized patients (normotensive group) and 21 hypotensive anesthetized patients (hypotensive group). IBP was measured in either the right or left radial artery by a Gould P23XL calibrated transducer while TBP was measured at the radial artery of the other side. The outputs of the two systems were recorded on a PCM tape recorder for later analysis. The TBP waveform was similar to the IBP waveform. A significantly high correlation existed between the two methods for systolic, mean and diastolic measurements. Magnitudes of bias (mean difference between the two methods) of the normotensive group were -0.4 +/- 5.9 (mean +/- SD) mmHg for systolic, -0.1 +/- 4.7 mmHg for mean and 0.0 +/- 5.4 mmHg for diastolic measurements and those of the hypotensive group were -0.7 +/- 6.6 mmHg for systolic, 0.7 +/- 5.0 mmHg for mean and 2.8 +/- 5.8 mmHg for diastolic measurements. The "limits of agreement" (mean difference +/- 2SD of bias) for both groups were within the range which would be acceptable for clinical setting. These results indicate that tonometry provides accurate, reliable and real-time blood pressure measurements together with continuous display of the pressure waveform for wide range of blood pressure. It was concluded that this system may replace the conventional invasive intra-arterial blood pressure monitoring system. This system is especially useful if it is combined with pulse oximetry and capnography.