- Background. Little information is available about the effect of antibiotic treatment on the prevalence and MIC of the subsequently isolated pathogens in cases of acute otitis media (AOM) failing a course of antibiotic therapy. This information is important, particularly regarding the effectiveness of the oral antibiotics used in children failing initial therapy. Patients and methods. One hundred eighty-one children with culture-positive AOM were prospectively studied between October, 1995, and July, 1996. Sixty-three (35%) patients received various antibiotics for variable periods during the 14 days preceding enrollment. Results. A total of 94 Streptococcus pneumoniae (Pnc) and 113 Haemophilus influenzae (Hi) were isolated. Thirty-eight Pnc and 35 Hi were isolated in the 63 patients with recently treated AOM. Pnc as a single isolate was more prevalent in patients recently treated with antibiotics (27 of 63, 43%) than among those not recently treated (32 of 118, 27%, P = 0.047). The MIC50 values of penicillin, cefaclor and cefuroxime axetil for Pnc were significantly higher in the pneumococci isolated from patients recently treated than among those isolated from patients not recently treated with antibiotics (0.38, 3 and 0.75 μg/ml vs. 0.094, 0.38 and 0.12 μg/ml, respectively). Seventynine percent of Pnc isolates in the recently treated group had MIC for penicillin of >0.1 μg/ml vs. only 47% in those not recently treated (P < 0.05). The respective figures for MIC >0.5 μg/ml of cefaclor were 79% vs. 41% for the recently treated and not recently treated groups (P < 0.001); cefuroxime MIC >0.5 μg/ml was found in 61 and 25%, respectively (P = 0.001). Conclusions. Pneumococcus is more prevalent in AOM after a recent antibiotic treatment, and the MIC of the commonly used beta-lactam drugs for Pnc is considerably higher in this setting. In view of our data, the use of oral cephalosporins like cefaclor or cefuroxime as second line drugs in the treatment of unresponsive AOM, particularly in regions where resistant PNC is prevalent, should be reconsidered.