After the development of computed tomography, the diagnostic importance of plain X-rays of paranasal sinuses has been reduced. But still the conventional plain X-rays of paranasal sinuses are used as a primary diagnostic tool for the evaluation of the paranasal sinuses. So we designed the study to evaluate the reliability of the conventional views in predicting the extent of disease in chronic sinusitis in comparison with ostiomeatal unit CT (OMU-CT). 70 cases were involved in the study for comparison of the PNS series and the OMU-CT in chronic paranasal sinusitis. The radio-opacity was classified into four groups in anterior and posterior ethmoid sinuses, frontal sinus, and sphenoid sinus, and eight groups in maxillary sinus according to the degree of aeration and radio-opacity. The diagnosis of plain films was classified into three categories (matched diagnosis, over-diagnosis, and under-diagnosis) in comparison with the radio-opacity depicted by corresponding OMU-CT. The incidence of matched diagnosis was as follows (in the order of percentages) : anterior ethmoid sinus (78.6%), sphenoid sinus (70.7%), frontal sinus (63.5%), posterior ethmoid sinus (60.2%), and maxillary sinus (53.0%). The incidence of over-diagnosis was highest in the maxillary sinus (33%) and lowest in the anterior ethmoid sinus (8.2%). The incidence of under-diagnosis was highest in the posterior ethmoid sinus (24.5%) and lowest in the sphenoid sinus (12.2%). And the incidence of the over-diagnosis or under-diagnosis was two times higher in the posterior ethmoid sinuses than in the anterior ethmoid sinuses. As a conclusion, the prediction of chronic sinusitis in the posterior ethmoid sinuses needs more careful interpretation than in the anterior ethmoid sinuses and if the status of maxillary sinuses is classified into a complex categories such as clear, minimal mucosal thickening, marked mucosal thickening, air-fluid level, cystic shape, nearly total hazziness, nearly total hazziness and air bubble like shape and total hazziness, the incidence of matched diagnosis was only about 50%. Therefor, nasal endoscopy should be performed with PNS series for relatively exact diagnosis of the disease of the anterior ethmoid sinus, so that increased matched diagnosis rate of the disease of the posterior ethmoid, maxillary, and frontal sinus may be obtained.