Background and objective: The technology of bispectral index monitoring is based on an algorithm developed from adults. This study was conducted to investigate the difference in bispectral index monitoring between paediatric and adult patients under intravenous anaesthesia. Methods: In all, 68 patients undergoing elective surgery were divided into three groups as follows: Group A (younger children, n = 20, 3 yr <= yr <= 5 yr), Group B (older children, n = 20, 6 yr <= yr <= 12 yr), Group C (adults, n = 28, 25 yr <= yr <= 50 yr). All patients were induced by plasma target-controlled infusion of propofol till loss of consciousness (loss of response to verbal commands and eyelash reflex). The plasma concentration (Cp-propofol) and effect-site concentration (Ce-propofol) of propofol and bispectral index values were compared at loss of consciousness and regaining of consciousness. During the operation, remifentanil was infused at a fixed rate of 0.25 mu g kg(-1) min(-1), and the minute average bispectral index values of three groups were also compared at different stable concentrations of propofol (Ce-propofol = Cp-propofol = 2, 3, 4 and 5 mu g mL(-1) respectively). Results: At loss of consciousness and regaining of consciousness, the bispectral index values in Group A (loss of consciousness: 74.7 +/- 6.3, regaining of consciousness: 81.1 +/- 10.5) were significantly higher than those in Group C (loss of consciousness: 68.6 +/- 9.4, regaining of consciousness: 74.5 +/- 6.0) (P < 0.01). There was no statistical difference between Cp-propofol in all groups at loss of consciousness and regaining of consciousness. The Ce-propofol at loss of consciousness in Group A (3.57 +/- 0.60 mu g mL(-1)) and B (3.25 +/- 0.44 mu g mL(-1)) were significantly higher than those in Group C (2.15 +/- 0.86 mu g mL(-1)) (P < 0.01). At the same stable concentrations of propofol, the bispectral index values in Group A and B were significantly higher than those in Group C, and the bispectral index values in Group A were also significantly higher than those in Group B (P < 0.01). The Ce-propofol when bispectral index <= 40 in Group A, B, C were approximately at 6, 5 and 3 mu g mL(-1), respectively. Conclusions: At loss of consciousness and regaining of consciousness, the bispectral index values of younger children group are significantly higher than those of adults. At the same stable concentrations of propofol, the bispectral index values are significantly different between children and adults. This study suggests that there is deviation when the adult algorithm of bispectral index monitoring is applied in paediatric patients under intravenous anaesthesia.