The PNS(paranasal sinus) radiography is the fundamental method to diagnose the lesion of sinuses. But we have little information abount the unilateral lesion on PNS. Among the 860 patients with plain PNS X-ray from May 1986 to April 1987, the 82 cases(9.2%) showing the unilateral lesion on PNS radiology were reviewed. The obtained results were as follows : 1) The causes of unilateral sinusitis were chronic nonspecific sinusitis(56.1%), retension cyst, maxillary cancer, aspergillosis, choanal polyp, inverted papilloma, mucocele, dental sinusitis, and tuberculosis in descending order of frequency. 2) The sexual distribution was 42.7% in males and 57.3% in females, showing a slight female preponderance. The unilateral sinus lesion was distributed in all age groups and the age distribution shows a significant difference among the various diseases. 3) The chief complaints of the patients were unilateral nasal obstruction, purulent rhinorrhea, postnasal drip, and headache in order of frequency,, and show some differences among the diseases. 4) The duration of symptom showed some variation according to each disease, but it was less than 1 year in 61% of all the cases. 5) The involved sinus was most commonly maxillary(97.6%), and in descending order of frequency, ethmoid, frontal, and sphenoid sinus. 6) The characteristics of nasal discharge contained purulent, mucoid, and bloody in decreasing order, and bloody discharge was noted only in the maxillary cancer and aspergillosis. 7) PNS CT(computed tomographic) scans taken in 22 cases of unilateral sinus lesion showed mucosal thickening in all cases. In maxillary cancer and aspergillosis, bone destruction and calcification were noted respectively. Above results suggest that the unilateral sinus lesion detected in plain PNS radiography should be suspected to have various diseases including non-specific inflammation. PNS CT scans can be regarded as useful method for differenciation of these various diseases.