Tonsillectomy and adenoidectomy are the most common operations in the ENT field. Recent emphasis on careful selection of patients for these procedures derives from concern for complications of surgery, as well as the immunologic and anatomic function of tonsils and adenoids. Adenoidectomy can relieve nasopharyngeal airway obstruction caused by enlarged adenoids, and can prevent complications such as heart failure and recurrent or chronic otitis media. We researched 117 cases of tonsilloadenoidectomy patients and 266 patients having routine examination from March, 1979 to February, 1981, and described the A/N ratio derived from linear measurements of lateral radiographs of the nasopharynx. The results of Impedance Audiometry and X-ray films from 117 cases of T & A patients were compared : 1) The average A/N ratio by age groups for 226 cases of normal children showed : 0.508 for age group, 0 to 3 years old, 0.533 for age group 4 to 6 years old, 0.524 for age group, 7 to 9 years old, 0.519 for age group, 10 to 12 years old, 0.507 for age group, 13 to 15 years old, 0.481 for age group, over 15 years old. The peak value lies in the age group of 4 to 6 years old. The sex difference of A/N ratio was not significant. 2) The average A/N ratio by age groups from 117 cases of T & A patient showed : 0.709 for age group, 0 to 3 years old, 0.733 for age group, 4 to 6 years old, 0.693 for age group, 7 to 9 years old, 0.707 for age group, 10 to 12 years old, 0.620 for age group, 13 to 15 years old, 0.756 for age group, over 15 years old. 3) Among the 117 cases of T & A patients, the average A/N ratio for 57 cases in abnormal tympanogram was 0.688 and the average A/N ratio for 60 cases in normal tympanogram was 0.705. 4) Among 57 cases in abnormal tympanogram : 6 cases(10.5%) showed 0.40-0.59 of A/N ratio, 44 cases(77.2%) showed 0.60-0.79 of A/N ratio, and 7 cases(12.3%) showed 0.80-0.99 of A/N ratio. 5) Among 117 cases of T & A patients, 3 of 15 cases showing A/N ratio under 0.6, and 67 of 102 cases showing A/N ratio over 0.6, had paranasal sinusitis.