To evaluate the incidence and significance of radiographic sinus opacification in infants, we performed computed tomography (CT) of the maxillary and ethmoid sinuses in conjunction with routine cranial CT in 100 infants from birth to 12 months of age. CT was performed for indications other than sinusitis. Prospective concurrent clinical history was obtained and physical examination of the upper respiratory tract was performed. Of 100 infants, 16 had hypoplasia of the maxillary sinuses; 81% (13/16) of these were less than 2 months of age. The antra showed progressive increase in size during the first year of life. Of the 100 infants, 70 had CT sinus opacification, including 67% of those without historical or physical evidence of upper respiratory tract infection. There was a positive correlation of CT findings between the maxillary and ethmoid sinuses in 80% of the infants older than 2 months of age but in only 49% of the younger infants. Radiographic sinus opacification in infants is of uncertain significance and is not diagnostic of upper respiratory tract infection, much less of sinusitis.