We studied middle ear pressure variations in 17 patients with mild upper airways or middle ear diseases, during nitrous oxide anaesthesia. We observed a significant increase in middle ear pressure during the administration of nitrous oxide, and a significant decrease after nitrous oxide was discontinued. Nine patients showed periodical reductions of the middle ear pressure during the inhalation of nitrous oxide, due to passive openings of the eustachian tube. In the remaining 8 patients (47%) this mechanism has not been effective. A comparison between these two groups shows significantly higher levels of middle ear pressure in the group of patients with abnormal function of the eustachian tube, both during and after the administration of nitrous oxide. Our results demonstrate that patients with mild upper airways or middle ear diseases are likely to show an impaired tubal function. Therefore, nitrous oxide should be used with caution in the presence of such diseases. Moreover, nitrous oxide is generally controindicated in middle ear surgery, as the pressure gradients between middle and external ear can jeopardize the success of the operation.