Servoplethysmomanometry (Penáz method) is based on the principle of "vascular unloading" or "arterial volume clamp". It permits continuous recording of the arterial pressure pulse in a finger. We evaluated such a device (Finapres) intraoperatively in cardiac surgical patients and compared the results with those obtained using simultaneous intra-arterial pressure recording. METHODS. Intravenous anesthesia was employed in a total of 31 patients. Invasive pressure monitoring was carried out in the radial artery. The cuff of the Finapres was wrapped around the 3rd finger, either on the same side as the radial cannula (group I, n = 15) or on the contralateral side (group II, n = 16). Quantitative comparison was accomplished for the paired values obtained by the two methods by calculating linear regression equations and correlation coefficients (r), as well as for the differences between the paired values (means, SD, frequency distributions). RESULTS. In group I (ipsilateral recording), r was 0.74 for systolic (SAP), 0.52 for diastolic (DAP), and 0.77 for mean (MAP) pressure. The means +/- SD of the differences (mm Hg) were -1.6 +/- 16.6 (SAP), 4.6 +/- 11.5 (DAP), and 0.2 +/- 11.3 (MAP). In group II (contralateral recording), r was found to be 0.86 (SAP), 0.72 (DAP), and 0.82 (MAP). The mean differences were -2.7 +/- 13.1 (SAP), 4.7 +/- 9.8 (DAP), and -2.6 +/- 10.9 mm Hg (MAP). Prior to cardiopulmonary bypass (CPB) the results of the two methods corresponded to a higher degree than after CPB; during CPB the MAP values correlated poorly in the two groups (r = 0.62). DISCUSSION. The results obtained may be interpreted to mean that the Finapres will enable us to monitor arterial blood pressure continuously with satisfactory reliability when the cuff design has been improved.