A prospective evaluation of the paranasal sinuses was performed on a consecutive series of 137 pediatric patients referred for cranial CT. Approximately one-half of the patients less than 13 years of age had some degree of maxillary or ethmoid sinus opacification. The prevalence and severity of opacification was approximately the same for the maxillary and ethmoid sinuses. Sphenoid sinus abnormality was less common (16% of patients) and was usually minimal or mild. No incidental frontal sinus abnormalities were observed. This study confirms previous reports, based on plain film radiography, of the prevalence of incidental maxillary sinus opacification in children. However, contrary to some prior studies, we did not find a relatively higher rate of opacification in children less than 1 year of age. This may be due to overdiagnosis of maxillary sinus opacification on plain films, in small children. The diagnosis of sinusitis in childhood must take into account not only the radiographic findings but clinical signs and symptoms. Correlation is needed to avoid overdiagnosis in patients referred for sinus radiography for nonspecific indications or who have incidental opacification noted on radiographic or CT studies of the skull and brain.