As Grocott pointed out in his editorial, processed electroencephalography (EEG) monitors such as the bispectral index (BIS) are increasingly being used [1]. In response to this editorial we present two cases with BIS values that contradict the equation ‘low BIS = less brain activity’, highlighting that certain patients’ characteristics may result in misleading BIS values, especially in the presence of low brain activity [2, 3]. The first case was a 60-year-old male, three months after frontal tumour resection, who presented for a ventricular peritoneal shunt. Pre-operatively he was cognitively impaired but clearly responsive. We did not prescribe any premedication. Because of miscommunication about the site of surgery, we applied the BIS to the left forehead, directly over a large fluid collection. The initial BIS reading before induction was 44. We then induced and maintained anaesthesia with propofol and fentanyl. As the patient lost consciousness the BIS readings did not decrease, but actually increased to above 60. Postoperatively and with the patient conscious, the BIS readings were again in the low 40s, confirming BIS’s completely inverse behaviour: according to the BIS, the patient was deeply unconscious while he was alert, and apparently susceptible to awareness while he was deeply unconscious. The second case was a 44-year-old male scheduled for a ventricular peritoneal shunt two months after traumatic brain injury and left frontal parietal craniectomy. He had been unresponsive since the day of the accident. Two BIS monitors were applied, one over each side of the forehead, showing similar, adequately high values before induction. After induction, both BIS values dropped, but interestingly, the BIS placed over the pathologic left side showed consistently higher values than the right side. According to the values obtained, the severely damaged brain tissue of the patient’s left side was less affected by anaesthesia than the right side. Or, in other words, only the intact right side was able to produce the EEG pattern identified by BIS as indicating adequate anaesthesia and lack of susceptibility to awareness. The exact BIS algorithm has not been published, and therefore explanations for this phenomenon remain speculative. Both patients suffered from extensive left frontal/parietal brain tissue loss, significantly reducing electrical output from neurons on one side of BIS placement. Therefore, the BIS could have relied more on electric signals from the face and eye muscles, which were unmasked on the pathologic side. Discrepancies of a lesser degree between simultaneously recorded BIS values on each side have been reported [4]. But further studies are needed in patients suffering from frontal lobe injuries to verify our observations of higher BIS values over damaged brain tissue and of inverse BIS values over damaged brain tissue after anaesthesia induction. Depth of anaesthesia remains a complex, and in some patients, probably an impossible assessment with our present monitors.