Objectives: The role of the viridans group of streptococci (Streptococcus oralis) in the prevention of colonization with Streptococcus pneumoniae was investigated in an adenoid organ culture system. Methods: The adenoids from 10 patients who were undergoing adenoidectomy for either hypertrophy or recurrent otitis media were used. Results: Streptococcus oralis Parker and S oralis Booth (two organisms isolated from the nasopharynges of patients undergoing adenoidectomy only and patients undergoing adenoidectomy and bilateral tympanostomy with tubes, respectively) uniformly inhibited both penicillin-sensitive and penicillin-resistant S pneumoniae. Although both strains of S oralis inhibited the growth of both S pneumoniae strains, strain Parker provided more complete inhibition than did strain Booth. Conclusions: The results indicate that some strains of S oralis may inhibit the growth of the most serious pathogens in the nasopharynx. It is therefore possible that colonization of inhibitory strains of viridans streptococci may be used in the nasopharynx as a relatively safe and inexpensive approach to prevention of recurrent otitis media in some children and of recurrent suppurative sinusitis in both children and adults. © 2006 Annals Publishing Company. All rights reserved.