A newly developed arterial tonometer enabled us to measure the blood pressure waveforms in addition to determining systolic and diastolic pressures noninvasively and continuously. Twenty-eight adult patients undergoing orthopedic surgery under controlled hypotension were studied. Systolic blood pressure was reduced to two-thirds of baseline values with an infusion of nitroglycerin during nitrous oxide/enflurane anesthesia. Intraarterial blood pressures were simultaneously measured in either the right or the left radial artery with a cannula and a Gould P23XL calibrated transducer; tonometric monitoring was performed on the contralateral radial artery using a Colin CBM-3000 instrument. The outputs of the two blood pressure measurement instruments were recorded for later data analysis. The shape of the tonometric pressure waveform was nearly identical to the waveform recorded intraarterially even during controlled hypotension. Regression analyses of 2039 paired tonometric and intraarterial blood pressure values during the hypotensive period showed good correlations (r= 0.78 for systolic, r= 0.81 for mean, and r= 0.70 for diastolic pressures). The accuracy of systolic, mean, and diastolic readings was from 4 to 7 mm Hg with negligible bias and did not differ significantly among six systolic, four mean, and four diastolic pressure groups. Our results indicate that arterial tonometry can provide accurate, reliable, and real-time monitoring of blood pressure even during controlled hypotension.