Isolates were obtained from 169 middle ear aspirates of 148 children with recently treated acute otitis media (RTOM) and from 123 aspirates of 84 children with previously untreated acute otitis media (UOM). In RTOM Streptococcus pneumoniae and Group A streptococcus (P less than 0.02) were recovered less frequently whereas Staphylococcus aureus (P less than 0.001) and Haemophilus influenzae (P less than 0.02) were recovered more frequently than in UOM. Patients with RTOM had more beta-lactamase-producing organisms and multiple isolates. Bilateral sterile aspirates were more common in RTOM although bilateral otitis media was more common in UOM. In RTOM most Branhamella catarrhalis isolates were co-pathogens whereas Staphylococcus epidermidis were always isolated in pure culture. Amoxicillin was the antimicrobial most frequently (83.9%) received for the recently treated episode of otitis media. In RTOM 62.4% of isolates were susceptible to previously prescribed antibiotics, suggesting that factors other than antimicrobial susceptibility are also important in the occurrence of RTOM.