Suppression ratio (SR) derived from bispectral index (BIS) monitoring is correlated to EEG burst suppression. It may occur during deep anaesthesia, but also in the case of metabolic or haemodynamic brain injury. The goal of the study was to describe the occurrence of SR and to determine factors associated with SR during propofol-remifentanil general anaesthesia maintenance.We conducted a post hoc analysis of BIS recordings in consecutive patients included in two multi-centre trials, undergoing non-cardiac surgery using a dual closed-loop BIS controller allowing automated propofol-remifentanil administration. The percentage of time spent with a BIS value between 40 and 60 (T(BIS 40-60)) was measured. Two groups of patients were defined: the SR group, including patients with at least one episode of SR value >10% lasting more than 1 min, and the control group. Factors associated with SR were analysed using a stepwise multivariate analysis.A total of 1494 patients [age=57 (17) yr; T(BIS 40-60)=76 (17%)] were analysed and 131 (8.7%) patients constituted the SR group. The main independent factors associated with SR were advanced age [odds ratio (95% confidence interval)=4.80 (1.85-12.43) (P=0.027), 10.59 (3.76-29.81) (P80 yr, respectively], history of coronary artery disease (CAD) [2.53 (1.47-4.37) (P=0.001)] and male gender [1.57 (1.03-2.40) (P=0.03)].The occurrence of SR during BIS-controlled propofol and remifentanil anaesthesia is mainly observed in elderly male patients or in patients with a history of CAD. The mechanisms underlying SR and the potential consequences for the patient's postoperative outcome remain unsolved.