BACKGROUND: Recent studies suggest that implicit memory (especially perceptual implicit memory) persists during adequate general anaesthesia in adults. Studies in children, however, have failed to demonstrate implicit memory during general anaesthesia, possibly because of differences in methodological design. We therefore designed a prospective study with the aim of evaluating implicit memory in children undergoing general anaesthesia, using a perceptual memory test based on the mere exposure effect, previously tested in a control group. METHODS: Twelve infrequent neutral words were played 12 times in a random sequence via headphones to 36 children aged 8-12 yr during elective or emergency surgery. The children were not premedicated, and general anaesthesia was maintained with isoflurane. The word presentation started immediately after the surgical incision. Within 36 h after the stimulus presentation, the memory was assessed by using a forced-choice preference judgement task. Time constraint and word deterioration with a low-pass filter were used to prevent the subjects from utilizing intentional retrieval. The implicit memory score was obtained by calculating the proportion of target words preferred, which was compared with the chance level (0.5). RESULTS: The percentage of correct responses given by the children was comparable with the chance level. The memory score was mean (sd) 0.48 (0.16) (95% CI 0.43-0.53). CONCLUSIONS: The use of a perceptual implicit memory test based on the mere exposure procedure in children failed to reveal any evidence of implicit memory under general anaesthesia.