Although there have been several anecdotal reports of the use of buccal pulse oximeter monitoring (Spo2) when digital Spo2 monitoring cannot be used, there have been no objective evaluations of the accuracy of buccal Spo2 monitoring. The purpose of this study was to systematically compare buccal Spo2 monitoring to both digital Spo2 and arterial O2 saturation monitoring (Sao2) in both generally anesthetized patients in the operating room (n = 31) and critically ill patients in the intensive care unit (n = 23). Buccal Spo2 probes were prepared by taping a malleable metal bar securely over the back of a Nellcor Oxisensor D-25 probe and bending the metal bar and buccal probe firmly around the corner of the patient's mouth. All buccal and finger Spo2 and Sao2 measurements were made simultaneously during hemodynamic stability. We found that buccal Spo2 was higher than finger Spo2 and agreed more closely with Sao2 for both patient groups (98.1% +/- 2.6%, 96.8% +/- 3.5%, 98.5% +/- 2.5%, respectively [mean +/- SD]). The operating room patients had higher buccal and finger Spo2 and Sao2 (99.3% +/- 1.5%, 98.9% +/- 1.4%, 99.5% +/- 0.7%, respectively) than the intensive care unit patients (96.4% +/- 2.9%, 94.1% +/- 3.5%, 96.6% +/- 3.5%, respectively). Although buccal Spo2 monitoring has several disadvantages (i.e., the probe requires preparation, can be more difficult to place, may be less readily accepted in awake patients, and is often mechanically dislodged during airway maneuvers), we conclude that buccal Spo2 monitoring is a more than adequate oximetry alternative when digital Spo2 monitoring is not an option (digits are unavailable or available digits are mechanically interfered with).