In order to clarify the role of the microflora of the nasopharynx and adenoids as causative factors of otitis media with effusion (OME), bacteriologic species in middle ear effusions (MEEs), nasopharyngeal smears, and adenoid tissues of children with OME were cultured. The change in the microflora of the nasopharynx after adenoidectomy, and the degree of bacterial agreement between the microflora of the nasopharynx and adenoid tissue were investigated. Nasopharyngeal bacterial flora were investigated in 259 children with OME from November 1984 to November 1987. The patients were divided into two groups: group A (43 patients) underwent adenoidectomy, and group B (216 patients) had no adenoidectomy. Hemophilus influenzae, Streptococcus pneumoniae, Branhamella catarrhalis, and Staphylococcus aureus were cultured in 66.7% of nasopharyngeal smears from group B, and in 46.5% from group A. Adenoid specimens for bacteriologic investigation were obtained from 38 adenoidectomy patients at surgery. Of these, H influenzae were cultured from adenoid specimens taken from 16 of the 26 patients with OME (group C), while only three of the 12 adenoidectomy patients without OME (group D) were found to have H influenzae in their adenoid specimens. The findings of this study suggest that adenoid vegetation plays an important role in the etiology of OME.