Adenoidal-nasopharyngeal ratios (AN ratios) obtained by simple linear measurements from lateral skull radiographs are described. The AN ratio reliably expresses adenoidal size and patency of the nasopharyngeal airway. The validity of the ratio as an indicator of adenoidal size was determined by evaluation of measurements of radiographs of 1,398 infants and children and comparison with a subjective visual assessment made by experienced observers in 92 patients. An AN ratio greater than 0.80 was present in 34 of 36 patients (94%) subjectively judged to have enlarged adenoids.