BACKGROUND: We can usually see the yawning at induction of anesthesia, however, it has not been studied as such an indicator of anesthesia. The yawning is one means of changing arousal level, and a sign or marker that such a change is occurring, although its functions are not well understood. The purpose of the present study was to evaluate the yawning whether it could be used as an indicator of induction of anesthesia, using its property as a marker of changed arousal level. METHODS: In 60 adult patients, group 1 was done propofol target controlled infusion (TCI) with Stelpump software, while group 2 was done thiopental TCI similarly. Clinical indicators of induction of anesthesia were measured as follows: loss of verbal control (LOV), loss of eyelash reflex (LOE), the yawning. In addition, the occurrence of apnea (OOA) were measured, too. We assessed the hypnosis levels of indicators of induction of anesthesia including the yawning and demonstrate their effect site concentrations and elapsed time. Furthermore, we compared the incidences of yawning and apnea between both groups. RESULTS: Clinical indicators of induction occurred in the order of LOV > LOE > the yawning > > OOA in both groups. With respect to BIS, the yawning showed the lowest BIS and the highest effect site concentrations except OOA in both groups. The incidence of the yawning in group 2 was higher than in group 1 (about 82 vs 63%). On the contrary, the incidence of apnea in group 1 was higher than in group 2 (about 79 vs 53%). CONCLUSIONS: As far as the yawning could be shown, we could observe, it approximated most closely to their clinical impression of the 'true' induction in terms of the hypnosis level and its effect site concentration.