Blood pressure, heart rate and perspiration were and still are the only clinical signs for recognizing the depth of balanced anaesthesia in combination with muscle relaxants. Even experienced anaesthetists sometimes have difficulties in recognizing the necessary depth of anaesthesia using these parameters and in relatively rare cases the very unpleasant symptom of awareness occurs. Nowadays, processed EEG monitoring (pEEG) is used scientifically and also clinically for exact controlling of general anaesthesia. We report on our first experience with this method in 21 patients who were anaesthetized with propofol, alfentanil and nitrous oxide and relaxed with atracurium and whose depth of anaesthesia was carefully controlled by pEEG using the pEEG Monitor of Drägerwerke. The following parameters were recorded continuously: Spectral Edge Frequency (SEF) 50, SEF 90, SEF 95 and power between 8 and 20 Hz in relation to the power between 0 and 4 Hz (delta ratio). In agreement with other investigators we found that a sufficient depth of anaesthesia could be expected when the SEF 50 ranged between 2 and 6 Hz (4.9 +/- 0.9), the SEF 90 ranged between 10 and 13.5 Hz (11.9 +/- 0.6), the SEF 95 ranged between 14 and 16 Hz (14.8 +/- 0.8) and the delta-ratio ranged between 0.7 and 1.4 (1.1 +/- 0.2). Using this anaesthetic technique and a premedication with benzodiazepine, SEF 90 SEF 95 are the best parameters for monitoring the depth of anaesthesia. In one patient blood pressure and heart rate remained constant intraoperatively, but she described intensive dreams postoperatively.(ABSTRACT TRUNCATED AT 250 WORDS)