Recently, it has been demonstrated that paranasal sinuses are an important site of nitric oxide (NO) production in the upper airways. The aim of this study was to evaluate the NO nasal concentration in children with acute maxillary sinusitis before and after treatment with antibiotic therapy. We performed NO nasal measurements in 16 children 4 to 13 yr of age with acute maxillary sinusitis and compared values with 16 age- and sex-matched healthy control subjects. The diagnosis of acute sinusitis was done by clinical signs and symptoms in addition to radiographic examination. NO nasal concentrations were measured by a chemiluminescence analyzer. Nasal NO steady state during oral breathing was recorded. The mean +/- SEM NO nasal concentration in children with sinusitis was 70 +/- 8.7 parts per billion (ppb) and increased significantly to 220 +/- 15 ppb (p < 0.001) after antibiotic therapy (amoxicillin/clavulanate). NO values after recovery from sinusitis were similar to those of healthy control subjects (245 +/- 15 ppb, p = NS). NO nasal measurements were also performed before and after antibiotic treatment in nine children 4 to 12 yr of age with symptoms of upper respiratory tract infection but no symptoms of sinusitis. In these children NO nasal levels were 249 +/- 32 ppb and did not change (p = NS) after antibiotic therapy. We conclude that during acute maxillary sinusitis the concentration of nasal NO is largely decreased, probably because of an impaired flow of NO from the paranasal sinuses, and that NO returns to normal levels after antibiotic therapy.