BACKGROUND Sugammadex is a modified γ-cyclodextrin with a novel mechanism of action for reversing the steroidal neuromuscular blocking agent rocuronium. Bispectral index (BIS) is an EEG-derived measure which can be sensitive to frontal electromyographic (EMG) artifacts. We compared BIS values before and after sugammadex or neostigmine neuromuscular block (NMB) reversal in patients with or without high EMG activity. METHODS During stable propofol/remifentanil anaesthesia and rocuronium-induced block, 48 patients were randomly allocated to receive sugammadex 4 mg kg(-1) or neostigmine 50 μg kg(-1)/glycopyrrolate 10 μg kg(-1), 10 min after the end of surgery. RESULTS Five minutes after sugammadex administration, mean BIS 50.1 (10.3) increased (P=0.018) to 61.7 (7.9) in 11 patients with high EMG activity. In contrast, BIS 49.3 (4.9) remained at 51.9 (5.4) in 13 patients who had no EMG activity. Fifteen minutes after neostigmine administration, mean BIS 51.9 (8.1) increased (P=0.007) to 63.9 (8.1) in 13 patients who had reappearance of muscle activity. However, in 11 patients who had no EMG activity, BIS 52.3 (7.4) remained at 53.3 (6.8). There was no significant difference between the sugammadex and neostigmine groups over time. CONCLUSIONS We have shown that reversal of NMB with sugammadex or neostigmine increased BIS values dependent on the presence of EMG activity. Thus, the effect of muscle activity reappearance during rocuronium NMB reversal spuriously increasing the BIS value should be taken into consideration when relying on BIS monitoring for evaluating propofol/remifentanil recovery.