The coronal and sagittal diameters of the tracheal air column were measured on posteroanterior and lateral chest radiographs of 808 patients with no clinical or radiographic evidence of respiratory disease. The 430 male and 378 female subjects were 10-79 years of age. Assuming a normative range that encompasses three standard deviations from the mean or 99.7% of the normal population, the upper limits of normal for coronal and sagittal diameters, respectively, in men aged 20-79, are 25 mm and 27 mm; in women, they are 21 mm and 23 mm, respectively. The lower limit of normal for both dimensions is 13 mm in men and 10 mm in women. Deviation from these figures reflects pathologic widening or narrowing of the tracheal air column. No statistically significant correlation was found between tracheal caliber and body weight or body height.