- Background. Previous limited data suggest that acute otitis media (AOM) caused by Streptococcus pneumoniae can present as a more severe disease than that caused by Haemophilus influenzae or Moraxella catarrhalis, as expressed by both tympanic membrane and systemic findings. Objectives. To evaluate the severity of disease and impact of various pathogens, age, disease history and previous antibiotic therapy in children with AOM by using a comprehensive clinical/otologic score. Patients and methods. The study group consisted of 372 children ages 3 to 36 months with AOM seen at the pediatric emergency room during 1996 through 2001. All patients had tympanocentesis and middle ear fluid culture performed at enrollment. Clinical status was determined by a clinical/otologic score evaluating severity (0 = absent to 3 = severe) of tympanic membrane findings (redness and bulging) and patient's fever, irritability and ear tugging. Maximal severity score was 15. Results. There were 138 (37%) H. influenzae, 76 (21%) S. pneumoniae, 64 (17%) mixed infections (H. influenzae + S. pneumoniae) and 94 (25%) culture-negative cases. The overall clinical/ otologic score was higher in culture-positive than in culture-negative patients (9.27 ± 2.75 vs. 8.38 ± 3.08, P = 0.01). When analyzed by age groups, this difference was significant only for the youngest age group (3 to 6 months, P = 0.05). The severity scores for AOM caused by H. influenzae and S. pneumoniae were significantly higher than in the culture-negative AOM when tympanic membrane redness and bulging were analyzed separately. No differences were recorded in clinical/otologic scores between different pathogens (9.49 ± 2.86, 9.03 ± 2.72 and 9.09 ± 2.54 for H. influenzae, S. pneumoniae and H. influenzae + S. pneumoniae, respectively). The mean clinical/otologic score was higher in culture-positive than in culture-negative patients without relationship to previous antibiotic treatment or number of previous AOM episodes. Conclusions. (1) The clinical/otologic score of culture-positive young infants was higher than that of culture-negative infants; (2) the severity of tympanic membrane redness and bulging were the most indicative factors discriminating between a bacterial and nonbacterial etiology of AOM; and (3) the use of a clinical/otologic score could not discriminate among various bacterial etiologies of AOM.