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Background: The purpose of this study was to determine the normal diameters of larger airways in the Persian population, since these demographic variables are essential for interpretation of chest CTs and/or plain X-rays., Material/methods: During a 6-month period, sagittal and coronal diameters of tracheas of all cases admitted for a chest CT to the radiology department of the National Research Institute of Tuberculosis and Lung Disease (NRITLD) were measured. The patients had to accept to take part in the study and had to fulfill the inclusion and exclusion criteria of the study. Ninety-nine percent confidence intervals (99% CI) were used to define the upper and lower limits of normal., Results: Two hundred subjects, including 132 men and 68 women aged 20-85 years, were studied. CORONAL AND SAGITTAL DIAMETERS OF TRACHEAS IN THE UPPER PART WERE AS FOLLOWS: 1.8±0.24 and 2.06±0.27cm for men, and 1.48±0.20 and 1.49±0.24 for women, respectively. For the lower part that was: 1.8±0.23, 1.86±0.27, 1.51±0.18, and 1.46±0.23, respectively. For the right and left main-stem bronchi the values were as follows: 1.16±0.17 and 1.02±0.22 for men and 0.93±0.13 and 0.81±0.13 for women, respectively., Conclusions: The values determined by us had a narrower range of normality than the ones found in the previous reports, so they will stay more friendly for interpretation of individual cases.