BACKGROUND: In a noisy hospital setting, it is not easy to induce hypnosis or sedation calmly. Although the noise stress has been neglected, it seems to disturb a patient's sleep or induction of sedation. Therefore, we tried to evaluate the effects of loud operating room (OR) background noise on bispectral index (BIS) during monitored anesthesia care (MAC) by using an audiometer and BIS monitor. METHODS: Thirty adult patients (ASA class I) were scheduled two times for nasal or dental procedures at an interval of two or three days. In a randomized, cross-over study design, we prospectively compared the BIS values according to the loudness of OR noise in two different depths of sedation during MAC. Propofol target controlled infusion (TCI) was started at a propofol target concentration (CT) 2.0 microgram/ml using a DiprifusorTM with flash mode until a BIS 80 and/or a modified Observer's Assessment of Alertness/Sedation (mOAAS) score of 4 (group 1), and BIS 75 and/or mOAAS score 3 (group 2) was obtained. We evaluated the effect site concentrations and the elapsed time and checked the BIS at 50, 80, 110, and 120 dB of sound pressure level (SPL) in both groups. RESULTS: The BIS at 80, 110 and 120 dB of SPL in group 1 was significantly increased compared to those at 50 dB (P < 0.05). Similarly, the BIS at 110 and 120 dB of SPL in group 1 was significantly increased compared to those at 80 dB (P < 0.05). The patients in their twenties were most susceptible to loud OR noise during sedation. CONCLUSIONS: The loud OR background noise might be possible to interfere with induction of sedation to a degree, which was more noticeable on light to moderate sedation than for deep sedation.