PURPOSE: To evaluate the prevalence of the opacification of paranasal sinuses and to correlate the prevalence and severity of the sinus opacification with presence of upper respiratory infection (URI) in infants and children using CT. MATERIALS & METHODS: We analyzed CT scans of 162 children aged under 16 who have no signs and symptoms of paranasal sinusitis. Both sides of maxillary and ethmoidal sinuses were evaluated. We scored from 0 to 3 according to the degree of soft tissue opacification of each sinus and then summed up the scores of each sinus. We divided the children into 5 groups according to their age. We paid particular attention to the following respects: 1) the prevalence of the opacification of the paranasal sinuses in each group; 2) the difference in the prevalence between the children with and without URI ;3) the correlation between the severity of the sinus opacification and the presence of URI. RESULTS: Of 162 children, one or more paranasal sinus opacification was noted in 76(47 %):31(65%) less than 1 year old;11(52%) between 1 and 2 years old;16(53%) between 2 and 6 years old ;15(28%) between 6 and 12 years old;and 3(33%) above 12 years old. In children less than 1 year old, no significant difference in the prevalence of the sinus opacification was found between URI-positive(71%) and URI-negative(58%) subgroups. In chilren between 1 and 12 years old, although the prevalence of the sinus opacification in URI-positive subgroups was much greater than that in URI-negative subgroup, statistically significant difference was noted only in children between 2 and 6 years old. As to the correlation between the severity of the sinus opacification and the presence of URI, these was a statistically significant difference in children between 2 and 6 years old and between 6 and 12 years old. CONCLUSION: Although the exact pathophysiology is not fully understood, the opacification of the paranasal sinuses is not an uncommon finding at CT in children without the signs and symptoms of sinusitis. We think that the clinical correlation is essential in determining the diagnosis and treatment plan in cases that the sinus radiographs or CT scans show the abnormal findings.