In an effort to develop a simple and accurate method to identify children in whom adenoidectomy might prevent otitis media, the ventilatory function of the Eustachian tube was assessed by a manometric technique. Nasal pressures during swallowing were also determined in some. The study group consisted of 23 children with otitis media in whom tympanostomy tubes had been inserted. All were judged clinically and roentgenographically to have prominent adenoids. Inflation-deflation Eustachian tube ventilation studies were obtained in 36 ears that remained intubated, aerated and dry both before and eight weeks after adenoidectomy. Fifteen of the 36 (42 percent) ears had improvement in Eustachian tube ventilatory function postadenoidectomy which was attributed to relief of extrinsic mechanical obstruction of the tube. In the remaining 21 (58 percent) ears in which Eustachian tube function was not improved, mechanical obstruction was not apparent preoperatively. The effect of nasopharyngeal pressures on a pliant Eustachian tube (Toynbee phenomenon) due to obstruction of the posterior nasal choanae by the adenoid mass was suggested as a possible cause of functional Eustachian tube obstruction. In several instances in which preadenoidectomy mechanical obstruction of the Eustachian tube was not demonstrated, the tube appeared to have been made more pliant by the operation. This increase in compliance of the Eustachian tube was attributed to loss of adenoid support of the tube in the fossa of Rosenmuller. From this study, preliminary recommendations for selection of patients for adenoidectomy are the following: 1. Eustachian tube ventilation function tests in a dry, intubated middle ear; 2. if extrinsic mechanical obstruction of the Eustachian tube is present and chronic inflammation is absent, adenoidectomy will probably improve Eustachian tube function. The surgical technique should include adequate removal of the adenoid tissue in the fossa of Rosenmuller; 3. if the Eustachian tube does not appear to be mechanically obstructed, the adenoids should not be removed unless functional obstruction is suspected due to obstruction of the posterior nasal choanae. Adenoid tissue within the fossa of Rosenmuller should not be removed when such circumstances exist; and 4. in the abscence of obstructive adenoids to the nasal choanae or Eustachian tube, adenoidectomy probably will not improve Eustachian tube function and could make it worse. A more rational and effective approach to adenoidectomy for the prevention of otitis media in children may be possible through this type of preoperative evaluation.