Adenoid vegetation is the most common disease of otolaryngologic diseases in children, and the adenotonsillectomy occupies the greater part of our operations performed at our department. The vegetated adenoid induced otitis media through obstruction of E-tube, and chronic rhinitis and sinusitis through obstruction of choanae. This is a study on the ratio of chronic maxillary sinusitis by vegetated adenoid and the therapeutic effect of chronic maxillary sinusitis after adenotonsillectomy in 120 cases who received adenotonsillectomy at the Dept. of E.N.T., Han-Yang Univer. Hospit. from March 1979 to August 1980. The results were as followings; 1) The sex incidence was 72 males and 48 females, and the age distribution was from 5yrs. to 13yrs.. 2) A size was divided into the large, medium and small vegetation groups by which the soft tissue of nasopharynx was occupied to the nasopharyngeal air-space in the skull true lateral view, and according to these method there were 35cases in large group, 78cases in medium group and 7cases in small group. The age incidences were 8cases(22.9%) large group, 6cases(7.7%) medium group in 6yrs., 9cases(25.7%) large group, 17cases(21.9%) medium group in 7yrs., 8cases(22.9%) large group, 16cases(20.6%) medium group in 8yrs., 4cases(11.5%), 16cases(20.6%) medium group in 9yrs., and 4cases(11.5%) large group, 12cases(15.4%) medium group in 10yrs.. 3) By the P.N.S. X-Ray there were resulted the 56cases(46.7%) normal sinus and 64cases(53.7%) sinusitis. The age incidence of sinusitis were 9cases(64.3%) in 6yrs., 18cases(69.3) in 7yrs., 14cases(53.9%) in 8yrs., 10cases(47.6%) in 9yrs., and 6 cases(33.3%) in 10yrs., and the involved side of sinusitis were 15cases(12.5%) in right, 15cases(12.5%) in left, and 34cases(28.3%) in both sinusitis. 4) Among the 35cases(29.2%) of large vegetation group, normal sinus was 9cases(25.7%) and sinusitis was 26cases(74.3%), and the involved side of sinusitis were 5cases(14.3%) in right, 8cases(22.9%) in left and 13cases(37.1%) in both sinusitis. 5) Among the 78cases(65.0%) of medium group, normal sinus was 40cases(51.3%) and sinusitis was 38cases(48.7%), and the involved side of sinusitis were 10cases(12.8%) in right, 7cases(9.0%) in left and 21cases(26.9%) in both sinusitis. 6) All 7cases of small group were revealed normal sinus. 7) Follow up study was done to the 43cases in sinusitis. 24cases(55.8%) of these were improved and 19cases(44.2%) were not improved. In the vegetated degree of adenoid, 16cases(57.1%) of 28 in large group and 8cases(53.3%) of 15 in medium group were improved.