The study reported here was a nationwide assessment of otitis media (466 patients with acute suppurative otitis media and 476 with chronic suppurative otitis media), sinusitis (447 with acute sinusitis and 426 with chronic sinusitis), acute tonsillitis (724 patients), and peritonsillar abscess (141 patients) performed between November 1998 and March 1999. Eighty university hospitals, 79 affiliated hospitals, and 103 general practitioners participated. Methicillin-resistant Staphylococcus aureus(MRSA) comprised 15.6% of the 786 isolated strains of S. aureus. MRSA was frequently detected in patients with suppurative otitis media, but was uncommon in those with acute tonsillitis or peritonsillar abscess, and it was more common in those who had already been treated than in those who had not, with a significant difference between the groups. Vancomycin (VCM) showed the highest antimicrobial activity against MRSA and no VCM resistance was detected. Penicillin-sensitive Streptococcus pneumoniae(PSSP), penicillin-intermediate-resistant S. pneumoniae (PISP), and penicillin-resistant S. pneumoniae (PRSP) accounted for 49.6%, 28.5%, and 21.9% of the 228 isolated strains of S. pneumoniae, respectively. PISP and PRSP were frequently detected in children aged 5 years or younger. beta-Lactamase was produced by 96 of the 100 strains (96%) of Moraxella (Branhamella) catarrhalis. The 281 strains of Haemophilus influenzae isolated consisted of 199 beta-lactamase-negative, ampicillin-sensitive (BLNASe) strains (70.8%), 65 beta-lactamase-negative ampicillin-resistant (BLNAR) strains (23.1%), and 17 beta-lactamase-producing strains (6.0%). BLNAR strains were frequently detected in pretreated patients. Of these 281 strains of H. influenzae, 214 had nontypable capsules. In conclusion, the major bacterial species showed resistance to beta-lactams, indicating that care should be taken when selecting an appropriate antimicrobial agent.