BACKGROUND: The bispectral index (BIS) has been used as an indicator of sedative state and has been considered to be related to anesthetic agents and noxious stimulus. In this study, we measured BIS, blood pressure (BP) and heart rate (HR) during induction of anesthesia (IA) with propofol, with and without midazolam pretreatment. METHODS: A study design was used in 20 ASA physical status 1 and 2 patients aged from 18 to 60 years undergoing short (2 h) operation times. In the control group (group 1, n = 10), propofol 12 mg/kg/h was infused continuously after propofol 2 mg/kg as an intravenous bolus for IA preceded by normal saline. In group 2 (n = 10), propofol 12 mg/kg/h was infused continuously after half-strength propofol 1 mg/kg as an intravenous bolus for IA preceded by 0.1 mg/kg midazolam 2 min before. Patients received intravenous propofol for IA over 40 seconds. During the infusion, vecuronium (0.15 mg/kg) was given 3 5 min before intubation. The assistant and controlled ventilation were maintained with 100% oxygen over 5 min, and continued until BIS decreased to 40 and intubation was called for. The BIS, BP and HR were measured 2 min after midazolam or normal saline infusion, 3 5 min after propofol with vecuronium and 1, 3 and 5 min after endotracheal intubation. RESULTS: The midazolam pretreatment produced transient decreases in systolic BP and the BIS. During IA with propofol, BP decreased 20% in both groups. BIS decreased significantly 5 min after endotracheal intubation. CONCLUSIONS: Midazolam pretreatment attenuated the cardiovascular response to intubation, so BIS is considered a good monitor as a predictor of hypnotic state during intravenous anesthesia.