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A prospective study was designed to assess the effect of anaesthesia, including nitrous oxide, on tympanometric results of children undergoing myringotomy and possible grommet insertion. 155 patients (310 ears) were examined with a pneumatic otoscope and had tympanograms performed within 1 hour of operation. The patients were then anaesthetized by a combination of intravenous and gaseous anaesthetic. Immediately before myringotomy, a tympanogram was repeated. In 13% of patients predicted to have fluid at admission, there was a change in their tympanogram after induction of anaesthesia, suggesting clearance of fluid. At myringotomy, these ears were dry. We surmise that there was displacement of fluid from the middle ear by nitrous oxide during the early stages of anaesthesia. Thus we feel that, if an otologist confidently expects to find an effusion at myringotomy, the presence of a dry tap should not change his management plan.