BACKGROUND: Propofol offers the advantages of rapid onset of action and recovery. However, the pharmacokinetics of propofol in children is different from that of adults. Therefore, infusion of propofol by target-controlled infusion (TCI) has same difficulties in use. Also, bispectral index has shown a close relationship with plasma concentration of propofol. In this study, we measured the effect site concentration comparable to Bispectral index (BIS) 50 during induction and the awakening concentration. Furthermore, we tried to demonstrate age-related differences. METHODS: Forty five premedicated (atropine 0.02 mg/kg) children (ASA class I) scheduled for elective surgery were assigned to one of three groups (Group 1: 1-5 years, Group 2: 6-10 years, Group 3: 11-15 years). After intravenous injection of lidocaine 0.5 mg/kg, a propofol infusion was started at a target concentration of 6 microgram/ml by using a Stelpump. Anesthesia was maintained with propofol 4-6 microgram/ml and 67% nitrous oxide in oxygen. During anesthesia induction, we checked the effect site concentration comparable to BIS 50 and plasma/effect concentration for awakening during an emergence period. RESULTS: The effect site concentrations comparable to BIS 50 were 5.7 +/- 0.5 microgram/ml (group 1), 4.9 +/- 0.8 microgram/ml (group 2) and 3.8 +/- 1.1 microgram/ml (group 3), so there are significant differences among the groups (P < 0.05)(Table 2). At the moment of awakening, the current/effect site concentration in group 1 (2.0 +/- 0.1 microgram/ml, 2.6 +/- 0.2 microgram/ml) was significantly higher than those of group 3 (1.6 +/- 0.2 microgram/ml, 2.0 +/- 0.30 microgram/ml). CONCLSIONS: The effect site concentration comparable to BIS 50 showed significantly age-related differences and was higher in younger children. The current/effect site concentration at awakening was highest in group 1.