INTRODUCTION: The Narcotrend is a new EEG monitor designed to measure the depth of anaesthesia based on a 6-letter classification from A (awake) to F (general anaesthesia with increasing burst suppression) and divided into 14 substages (A, B0-2, C0-2, D0-2, E0,1, F0,1). This study was designed to investigate the assessment of the depth of anaesthesia as measured by the Narcotrend in comparison with bispectral index monitoring (BIS). METHODS: Both BIS and Narcotrend EEG electrodes were positioned on the patient's forehead as recommended by the manufacturers. All patients were premedicated with diazepam 0.15 mg/kg orally in the evening and on the morning before surgery. Induction of anaesthesia was started with a remifentanil infusion at 0.4 microgram/kg/min; 5 min later propofol was given for hypnosis using a target-controlled infusion initially started at 3.5 micrograms/ml. After loss of consciousness patients received 1.2 mg/kg of suxamethonium. Immediately after intubation, remifentanil was reduced to 0.2 microgram/kg/min, and the depth of anaesthesia was adjusted according to clinical needs by regulating the propofol target-controlled-infusion. BIS values and Narcotrend stages were recorded as data pairs in intervals of 1 min during anaesthetic induction and emergence, and in intervals of 5 min during maintenance of anaesthesia. RESULTS: Fifty patients undergoing orthopaedic surgery were studied and 2031 data pairs were obtained. An increasing depth of anaesthesia as indicated by the Narcotrend was associated with significantly lower mean BIS values. With BIS values between 100 and 85 (representing awake patients), 95.5% of all data pairs indicated a Narcotrend stage A or B. In case the BIS was found to be 65-40 (representing general anaesthesia) the corresponding Narcotrend stages were measured as D (52.4%) or E (41.1%). No patient complained of intraoperative recall when interviewed on the 1st and 3rd postoperative day. CONCLUSIONS: We could demonstrate that an increase of the hypnotic component of anaesthesia as indicated by BIS is accompanied by corresponding effects as displayed by the Narcotrend during propofol-remifentanil anaesthesia. The Narcotrend stages D or E are assumed equivalent to BIS values between 64 and 40 indicating general anaesthesia.