The bacteriologic characteristics of subacute maxillary sinusitis have not been delineated in the pediatric age group. Forty children between the ages of 2 and 12 years with respiratory symptoms for at least 30 but less than 120 days were evaluated. Nasal discharge and cough were the most prominent symptoms. Common radiographic findings were diffuse opacification and mucosal thickenings. Sinus aspiration was performed on 52 sinuses of 40 children. Bacterial colony counts greater than or equal to 10(4) colony-forming units per milliliter were found in 30 (58%) of 52 sinus aspirates obtained from 26 (65%) children. The bacterial species most commonly recovered were Streptococcus pneumoniae, Haemophilus influenzae, and Branhamella catarrhalis. Twenty-five percent of the maxillary sinus isolates were beta-lactamase producing; however, many of these were recovered from patients who had recently received antimicrobial therapy. Subacute and acute maxillary sinusitis are similar in regard to causative organism, clinical presentation, and radiographic findings.