A series of 100 patients with chronic maxillary sinusitis, treated at the department of otolaryngology, Seoul National University Hospital from January 1 to December 31, 1986 was reviewed for the diagnostic accuracy of the X-ray tomography compared to that of the plain X-ray. Radiological findings were interpreted as normal, mucosal thickening, partial haziness, total opacification, air-fluid level and polypoid change. Confirmatory findings by maxillary sinuscopy and radical antrum operation were classified as normal, mucosal thickening, single polyp or retention cyst, polyposis and maxillary empyema. The accuracy of tomography/plain X-ray in the cases of interpretation of ""normal"" was 76.9/77.8(%), in the cases of the ""mucosal thickening"" 73.5/68.6(%), in the cases of the ""partial haziness""(62.5/76.2(%), and in the cases of the ""total opacification"" 72.7/31.9(%). The polypoid change of the maxillary sinus mucosa was interpreted by tomography, in the accuracy of 82.6%. Significant difference in the diagnostic accuracy was present between plain X-ray and tomography, especially in the cases of mucosal thickening, polyposis, and empyema. Routine tomographic examination seems to be advisable in the patients with chronic maxillary sinusitis.