This study compared the relative effectiveness of two antimicrobial preparations, amoxicillin and amoxicillin-clavulanate potassium (Augmentin), in the treatment of acute maxillary sinusitis in children 2 to 16 years of age. Of 171 children with persistent (ten to 30 days' duration) nasal discharge or daytime cough or both, 136 (80%) had abnormal maxillary sinus radiographs. These children were stratified by age and severity of symptoms and randomly assigned to receive either amoxicillin, amoxicillin-clavulanate potassium, or placebo. After the exclusion of 28 children with throat cultures positive for group A Streptococcus and 15 who did not complete their medication, the remaining 93 children were evaluated: 30 received amoxicillin, 28 received amoxicillin-clavulanate potassium, and 35 received placebo. Clinical assessment was performed at three and ten days. On each occasion, children treated with an antibiotic were more likely to be cured than children receiving placebo (P less than .01 at three days, P less than .05 at ten days). The overall cure rate was 67% for amoxicillin, 64% for amoxicillin-clavulanate potassium, and 43% for placebo.