Adenoid is a kind of tonsil located in the posterior wall of the nasopharynx. Enlargement of the adenoid can produce obstruction of the nasopharynx and eustachian tube. Distrubance in discharge of nasal and paranasal secretions can be a cause of chronic rhinitis, sinusitis and otitis media. The diagnosis of enlarged adenoid by inspection is difficult due to its location. In the lateral roentgenogram of the skull the anterior wall of the adenoid is sharply delineated by air in the nasopharynx. The authors measured the sizes of adenoid and nasopharynx and calculated the adenoid nasopharyngeal ratio (AN ratio) from 1000 simple skull lateral roentgenograms of the children between the age of 0 to 16 years. Adenoidal size is gradually increasing in the childhood up to 9 years of age but almost unchanged in the older age group. The An ratio is highest in the age group of 8-9 years. In the age groups above 9 years of age the AN ratio is gradually decreased due to atrophic changes of the adenoid.