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Trimethoprim-sulfamethoxazole (T/S) has often been used as first and second line of treatment for acute otitis media (AOM). Because of the increasing resistance of Streptococcus pneumoniae and Haemophilus influenzae to T/S, we undertook the present study to investigate the bacteriologic and clinical efficacy of this drug in AOM.Fifty-four culture-positive evaluable patients ages 3 to 32 months with AOM were treated with T/S 4/20 mg/kg in two divided daily doses for 10 days. Middle ear fluid (MEF) was cultured at enrollment (Day 1) and on Days 4 and 5 after initiation of treatment. Additional MEF cultures were obtained if clinical relapse occurred. Clinical failure was determined when the symptoms and signs of AOM did not improve or recurred during therapy. Bacteriologic failure was defined by positive culture on Days 4 and 5, or negative on Days 4 and 5 but positive again before the end of treatment. Patients were followed until Day 28 ± 2.A total of 67 organisms were isolated from MEF specimens of the 54 study patients:S. pneumoniae, 24;H. influenzae, 40; and Streptococcus pyogenes, 3. Fifteen (63%) of 24 S. pneumoniae were nonsusceptible to T/S (trimethoprim MIC, >0.5 μg/ml), of which 10 (67%) were highly resistant to T/S (trimethoprim MIC, ≥4.0 μg/ml). Twelve (30%) of 40 H. influenzae and all 3 S. pyogenes isolates were nonsusceptible to T/S (MIC ≥ 4.0 μg/ml). Bacteriologic eradication occurred in 9 of 9 (100%) and 27 of 27 (100%) T/S-susceptible S. pneumoniae and H. influenzae, respectively, vs. 4 of 15 (27%) and 6 of 12 (50%) T/S-nonsusceptible S. pneumoniae and H. influenzae, respectively (P < 0.001). The 3 patients with S. pyogenes failed bacteriologically. Nine new organisms, not initially isolated, emerged during treatment, 7 of which (77%) were resistant to T/S. Altogether bacteriologic failure (organisms not eradicated plus newly emerged) occurred in 29 (53%) of 54 patients. Clinical failures occurred in 8 (15%) of 54 patients, and in 7 of these 8 cases the clinical failures occurred in those with bacteriologic failures. Ten patients relapsed clinically after completion of treatment and in 8 of them tympanocentesis for MEF culture was performed. Six of these 8 cultures were positive, and the initial pathogen was isolated in 4 of 6 (67%).A high bacteriologic failure rate as well as a considerable clinical failure rate occurred among patients with AOM treated with T/S. We believe that T/S is no longer an appropriate empiric choice for the treatment of AOM in regions where high T/S resistance among respiratory pathogens is reported.