A clinicostatistical study on chronic paranasal sinusitis was carried out on 3,868 patients with chronic paranasal sinusitis who had visited Seoul National University Hospital during the 7 year period from January, 1965 to August, 1971. Also the authors have analyzed 181 cases of chronic paranasal sinusitis who received surgical treatment during the same period. The study includes the patient's history of illness, blood pictures, X-ray findings, peration findings and histopathologic examinations and also have analyzed the status of the distribution of out- and in-patients during the above period. The results are as follows. 1) The percentage of patients with chronic paranasal sinusitis of the total number of out- and in-patients appeared to be decreasing progressively since 1965. As a whole the percentage of patients with chronic paranasal sinusitis of the total number of outpatients was 11.9% and of inpatients was 17.2%. 2) The percentage of patients who received surgical treatments of the total, outpatients was 10.9%. There was a increasing tendency to treat the patient surgically dispite of progressive decrease in number of patients with chronic paranasal sinusitis. 3) Greatest number of outpatients was visiting in August and the next was from January to May. 4) The sex ratio, male vs female, was about 6 : 4. 5) The most frequent occurrence of sinusitis was between 15 and 29 years of age, occupying 66.2% of outpatients, about 76% of inpatients. 6) The most frequent duration of illness was between 3 and 5 years periods. 7) The most frequent complaints were headache, rhinorrhea and nasal obstruction in between 80~76% respectively and much more female patients were complaining of headache. 8) Of the patients who received Caldwell-Luc's operation 6.1% had past history of receiving Caldwell-Luc's operation, and 2.8% intranasal antrostomy or sinus puncture irrigation. Thus, 8.8% of patients who received Caldwell-Luc's operation had the past history of having some kinds of operations. 9) The average value of hemoglobin in patients with chronic paranasal sinusitis was 1.2~1.3 g/dL lower than in normal population and the average value of ESR was higher. An average postoperative value of hemoglobin was 1.2 g/dL lower than preoperative value and the value of ESR was increased in more than 2 times. 10) In gross findings, mucosal thickening was about 60% and polyps between 20~28%, so they were the most important gross findings of the chronic paranasal sinusitis. On the other hand, of the patients who received sinus operations under the clinical diagnosis of chronic paranasal sinusitis, 2.4% of the ethmoid sinuses showed normal mucosa. 11) The majority of cases who showed any changes of the facial wall of the maxillary sinus had the thickenings of the bony walls. 12) The percentage of cases which showed pararell relationships between X-rays and operation findings was between 25.0~72.5% in cases of maxillary sinusitis and between 20.5~60.6% in cases of ethmoid sinusitis and the compatibility was more evident in cases of maxillary sinusitis. And as the pathologic changes in both X-rays and operation findings were more marked, the compatibility was also more evident. On the other hand, the degree of severity noted at the time of operations was more marked than that indicated by the X-ray findings. 13) The most frequent histopathologic findings were diffuse small round cell infiltrations, epithelial denudations or erosions, and diffuse fibroses. Edematous changes, atrophies of the mucosal glands, eosinophil infilations and mucosal retention cysts were also frequent. The most frequent pathologic diagnosis was chronic non-specific sinusitis in 50% ; the next was chronic active sinusitis in 25%.