PURPOSE: To assess the normal range of transverse and AP diameter of the trachea on simple chest radiographs and to determine whether or not there is any correlation between tracheal diameter and age, sex, height, or bodyweight. MATERIALS AND METHODS: Six hundred and ninety patients with no lesion on chest radiographs and noclinical evidence of respiratory disease were involved in this study. To obtein transverse and lateral diameters,the internal diameter of the tracheal air column was measured at a level 2 cm above the top of the aortic arch onboth posteroanterior and lateral radiographs. RESULTS: The normal ranges of AP and transverse diameters of thetrachea were 16 to 25mm, and 14 to 22mm in men, respectively and 12 to 20mm and 12 to 18mm in women. Statistically significant differences were observed between AP and transverse diameter in both in men and women, the former being consistently larger than the latter in both sexes. In men, significant correlations were observed between transverse diameter and patients' height, and between AP diameter and age as well as height. In women, significant differences were observed between AP diameter and patients' height, and transverse diameter and height as well asbody weight of patients. CONCLUSIONS: Normal tracheal diameter was larger in men than in women. and AP diameter was larger than transverse diameter. Patients' height showed persistent correlation with luminal diameter.