Although many studies have recently been conducted to elucidate the etiology of otitis media with effusion, investigators have as yet been unable to obtain a definite answer. We performed bacteriological examinations and clinical observations on 100 patients with otitis media with effusion in whom there was accumulation of middle ear fluid. We also conducted experiments on rabbits, in which stenosis of the pharyngeal orifice of the eustachian tube was induced mechanically to simultate the pathological condition found in otitis media with effusion. Bacteriological examinatons were made and the histological changes in the mucosal epithelium of the middle ear observed in 50 animals. The results can be summarized as follows: 1. Bacteria were detected in the accumulated fluid in 48 of 100 patients (48%) with otitis media with effusion. 2. The bacteria found in the middle ear fluid tended to correspond to the epipharyngeal bacteria in those patients with otitis media with effusion who had histories of recent acute infection. 3. Bacteria were detected in the middle ear fluid in adults with otitis media with effusion, as in children. Anaerobes were detected in 3 of 5 adults. 4. The rate of detection of bacteria was lower in patients with retraction of the tympanic membrane at the pars tensa and pars flaccida than in patients with retraction at the pars tensa only. The former condition was believed to be a more advanced state than the later. 5. Of the patients with otitis media with effusion, 66% had associated chronic sinusitis. 6. There were no differences in the rate of detection bacteria according to the properties, viscosity, or serosity of the middle ear fluid. 7. In aimal experiments with rabbits, the bacteria detected in the middle ear fluid tended to corresond to those found in the epipharynx for up to 30 days after treatment. 8. In regard to the changes in the mucosal epithelium in the middle ear in rabbits, the squamous epithelium changed to cuboidal epithelium, became ciliated, and changed to columnar epithelium with the passage of time. The above findings suggested that in otitis media with effusion, inflammation spreads to the middle ear cavity from such conditions as upper airway infections.