Data collected from 1980 to 1989 by investigators at the Pittsburgh Otitis Media Research Center were examined to detect changes over time in the prevalence of bacteria isolated from middle ear effusions in patients with otitis media. The organisms isolated most commonly from the 7396 effusions cultured at the center were Streptococcus pneumoniae and Haemophilus influenzae. S. pneumoniae predominated in the subgroup of patients with acute otitis media, whereas H. influenzae was isolated most frequently from patients with otitis media with effusion. The most notable changes to occur during the 10-year period were a statistically significant increase in the prevalence of S. pneumoniae in patients with acute otitis media and a progressive rise in the percentage of beta-lactamase-producing strains of H. influenzae and Moraxella (Branhamella) catarrhalis. The latter finding suggests the need for therapeutic alternatives to amoxicillin, which is not active against beta-lactamase-producing organisms, when these organisms are suspected or cultured from the middle ear.