This study was designed to explore the effect of nitrous oxide (N2O) on the amount of middle ear effusion. Seventy-six children referred for adenoidectomy or tympanostomy tube placement were divided into two groups in the basis of the method of anesthesia. One group of 39 children was ventilated with a mixture of 30% oxygen and 70% nitrous oxide, while the other group of 37 patients was ventilated with a mixture of oxygen and air. The amounts of middle ear effusion obtained in myringotomy were weighed and compared between these groups. Preoperative and perioperative tympanograms were performed. Ventilation with nitrous oxide caused a distinct rise in middle ear pressure. The amount of the middle ear effusion, however, remained the same in the two groups. It is concluded that the operating surgeon can rely on the myringotomy finding even when nitrous oxide anesthesia is used.