Acute otitis media (AOM) has been associated with epidemic peaks of respiratory syncytial virus (RSV) infection. RSV alone or in combination with bacteria has been the most frequently identified viral agent in middle-ear aspirates from patients with AOM. The apparently increasing prevalence of bacterial resistance to antimicrobial drugs, which has likely been fostered by selective pressure resulting from extensive use of the drugs, prompts curtailment of antimicrobial therapy when possible. We investigated the incidence and etiology of AOM in a group of children with bronchiolitis to determine whether AOM in such children might be due to RSV only. Subjects were children aged 2-24 mo who had bronchiolitis defined by strict clinical criteria, with or without AOM defined as middle-ear effusion plus ≥1 indicator of acute inflammation (otalgia, distinct erythema, bulging of the tympanic membrane). In patients with AOM, middle-ear aspirates were collected by tympanocentesis at study entry prior to administration of antimicrobials. Patients without AOM were re-examined at 48-72 hr and at 2-3 wk. Thirty-eight children with bronchiolitis were enrolled. Eighteen (47%) patients had AOM at study entry--12 unilateral and 6 bilateral. Four (11%) additional patients developed AOM during the 2-week follow-up period--3 unilateral and 1 bilateral. Tympanocentesis was performed in 28 of 29 ears from 21 patients with AOM. All patients with AOM had bacterial pathogens isolated from one or both middle-ear aspirates. Of 28 middle-ear aspirates, S. pneumoniae was isolated in 14 (50%), M. catarrhalis in 7 (25%), H. influenzae in 5 (18%), and S. aureus in 2 (7%). Two middle-ear aspirates yielded 2 pathogens each; 2 aspirates had no growth. One (7%) of 14 S. pneumoniae isolates had intermediate penicillin resistance; 7 (100%) of M. catarrhalis isolates and 1 (20%) of 5 H. influenzae isolates produced β-lactamase. We conclude that AOM is highly prevalent in patients with bronchiolitis, that most episodes develop early in the course of the illness, and that bacterial infection in such children is the rule. Curtailment of antimicrobial therapy on an assumption that bronchiolitis-associated AOM is exclusively viral in etiology is not warranted.