The Finapres (FIN) is a new noninvasive blood pressure monitor that provides continuous arterial waveform display with the use of a finger cuff. The authors assessed the accuracy of FIN mean arterial pressure (MAP) measurements relative to simultaneous direct radial arterial pressures in 20 patients undergoing general anesthesia for major elective surgery. Data were collected digitally with the use of RS-232 communications over a total of 16.2 h. The data were processed into 6012 interference-free time samples, each spanning 6 s. The authors determined the difference between FIN and direct MAPs during each time sample. The authors calculated not only the bias of FIN measurements, but also the frequency, magnitude, and duration of discrepancies between simultaneous FIN and direct MAPs. The overall bias of the FIN MAP was -0.5 +/- 1.0 mmHg, which was not significantly different from zero. However, 32.3 +/- 6.2% of all MAP comparisons differed by greater than +/- 10 mmHg, and 5.0 +/- 1.1% differed by greater than +/- 20 mmHg. Moreover, there was an average of one episode every 2 patient-hours when the FIN MAP differed by greater than +/- 20 mmHg for more than 1 min. Although the MAP measured by FIN accurately reflected direct MAPs most of the time, there were occasional discrepancies of different magnitude such that clinical usefulness may be limited in patients in whom continuous accurate blood pressure measurements are essential.