PURPOSE: To evaluate the changes and normal ranges of the artery-bronchus ratio (ABR) during respiration MATERIALS AND METHODS: We analyzed HRCT of 10 healthy adults. The HRCT findings of ten healthy adults were analysed. CT scanning was performed with 1 mm collimation at 3 mm intervals during full inspiration and full expiration, with a range during inspiration from 2 cm to 4 cm above the carina and from 4 cm above to 2 cm below the right hemidiaphragm. The range during expiration was from 1 cm to 3 cm above the carina and from 4 cm above to 2 cm below the right hemidiaphragm. ABiR (defined as the diameter of pulmonary artery divided by the inner diameter of the bronchus), ABoR (defined as the diameter of pulmonary artery divided by the outer diameter of the bronchus) and BLR (defined as the inner diameter of the bronchus divided by the outer diameter of the bronchus) were measured on the display console. RESULTS: The mean inner diameter of the bronchi was 2.04±0.73 mm during inspiration and 1.68±0.51 mm during expiration, while the mean diameter of the arteries was 3.95±1.03 mm during inspiration and 4.37±1.09 mm during expiration. The diameters of the bronchi were thus seen to increase during inspiration, and the diameters of the pulmonary arteries, to decrease. The mean thickness of the bronchial wall was 1.07±0.19 mm during inspiration and 1.06±0.24mm during expiration; thus, no change in thickness was seen during respiration (p<0.05). Mean ABiR was 2.01±0.60 (range 1.15-4.58) during inspiration and 2.59±0.74(range 1.16-4.9) during expiration, and in all cases the inner diameter of the bronchus was less than that of the accompanying pulmonary artery. Mean ABoR was 0.91±0.19 during inspiration and 1.09±0.22 during expiration. while for BLR, the corresponding fingures were 0.46±0.06, and 0.44±0.09. CONCLUSION: HRCT is a useful tool for evaluating changes in the pulmonary arteries and bronchi during respiration.