A Clinical Practice Guideline for the Diagnosis and Management of Acute Otitis Media in Children, in view of the causative organisms of the disease and their drug susceptibility, was issued in March 2006. In the guideline, cefditoren pivoxil (CDTR-PI, Meiact MS fine granules 10% for pediatric use) is recommended as an oral cephem antibiotic for the treatment of the disease. To collect information on the appropriate use of the drug in the clinical setting after issuance of the guideline, we conducted a specific postmarketing study of CDTR-PI in pediatric patients with acute otitis media. With this study, 2144 patients were enrolled in 305 medical institutions. Of them, 2006 and 1958 patients were chosen for safety and efficacy analysis, respectively. The incidence of adverse drug reactions was 1.79% (36/2006 patients). No unexpected or serious adverse drug reactions were reported by this study. The most common adverse drug reaction was diarrhea, which was reported in 26 cases (1.30%). The symptom resolved or subsided during CDTR-PI therapy or after discontinuation or completion of the therapy in all cases. The incidence of diarrhea in patients treated with CDTR-PI at 1.5- to 2-fold the usual dose was 2.70%, which was slightly higher than the usual dose, but that in patients more than 2-fold the usual dose was 1.92% which was not higher than 1.5- to 2-fold the usual dose. The incidence of diarrhea itself was not substantially high. Concerning the clinical efficacy of CDTR-PI, the response rate was 93.5% (1831/1958 patients). Among 1217 strains from whom 832 patients were detected as causative organisms at baseline bacteriological examination, the response rate by causative organism was 89.7% for Streptococcus pneumoniae, 90.3% for Haemophilus influenzae, and 92.2% for Moraxella catarrhalis. Among documented eradication of 577 strains with 427 patients, the bacterial eradication rate by causative organism was 83.3% for S. pneumoniae, 87.1% for H. influenzae, and 88.9% for M. catarrhalis. For each major resistant strain, the response rate was 88.0% for penicillin-intermediate S. pneumoniae (PISP), 90.1% for penicillin-resistant S. pneumoniae (PRSP), and 92.5% for beta-lactamase negative ampicillin-resistant H. influenzae (BLNAR), while the bacterial eradication rate was 85.7% for PISP, 77.5% for PRSP, and 81.8% for BLNAR. In addition, 457 patients "without myringotomy" and "tympanic swelling or otorrhea rated as a severity score of 8" (symptoms emphasized in the guideline) were selected as a subpopulation allowing us to define the dose-efficacy relationship more clearly. In this subpopulation, the relationship between the dose and the efficacy of CDTR-PI was assessed. The assessment revealed that the response rate was significantly higher in patients with a mean daily dose of at least 13.5 mg/kg than in those with a mean daily dose below 13.5 mg/kg. In summary, CDTR-PI raised no noteworthy concerns about its safety or efficacy in pediatric patients with acute otitis media. These findings reconfirm the usefulness of the drug.