BACKGROUND: The Qnox index is a novel monitor to quantify intraoperative nociception based on an electroencephalographic algorithm. We evaluated the ability of the Qnox index to discriminate noxious from non-noxious stimuli, respond to stimuli, and discriminate different levels of analgesia in patients under propofol anaesthesia with neuromuscular block. METHODS: Qnox was compared with heart rate and mean arterial pressure with five designated stimuli: tetanic stimulations without (tetanic 1) and with sufentanil (tetanic 2), skin incision, tracheal intubation, and a non-noxious period. The response around the skin incision was also evaluated at two target remifentanil concentrations. RESULTS: In 83 adult patients scheduled for elective surgery, Qnox performed worse than heart rate and mean arterial pressure in discriminating tetanic 2, tetanic 1, skin incision, and tracheal intubation noxious stimuli from the non-noxious period, with an area under curve of 0.52 (95% confidence interval 0.43-0.61), 0.54 (0.45-0.62), 0.67 (0.58-0.75), and 0.65 (0.57-0.73), respectively. The post-stimulus values of Qnox increased significantly after tracheal intubation and skin incision, but not after tetanic 1 or tetanic 2. Qnox values after skin incision were similar between the low- and high-remifentanil-concentration groups. CONCLUSIONS: Qnox had a poor ability to discriminate noxious stimuli from non-noxious stimuli. Although Qnox responded to tracheal intubation and skin incision, it did not respond to tetanic stimulations and failed to discriminate different levels of analgesia. The Qnox index was not superior to heart rate or mean arterial pressure in assessing nociception during general anaesthesia. CLINICAL TRIAL REGISTRATION: Chinese Clinical Trial Registry (ChiCTR2100046063).