Maxillary sinus aspiration and quantitative culture of the aspirate were performed in 50 patients, ranging in age from 1 to 16 years, with clinical and radiographic evidence of acute sinusitis. Of 79 sinuses aspirated, at least one was found to be infected in 35 (70%) children. Streptococcus pneumoniae, Branhamella catarrhalis, and Haemophilus influenzae were the most common organisms recovered. All H. influenzae were nontypeable. Twenty percent of the H. influenzae and 27% of the B. catarrhalis organisms were beta-lactamase positive and amoxicillin resistant. The subjects received either amoxicillin or cefaclor at a dose of 40 mg/kg/day in three doses for 10 days. The clinical cure rate with amoxicillin was 81%, compared to 78% with cefaclor. Radiographic improvement was similar in both treatment groups. Antibiotic therapy failed in four patients; three had been given amoxicillin, and one cefaclor. In three of these, a beta-lactamase-positive antibiotic-resistant bacterial species was recovered from the maxillary sinus aspirate; the fourth aspirate was sterile.