Object : To evaluate changes in the diameter of bronchi and pulmonary arteries during respiration. Materialsand Methods : The ratio of the diameter of matched inner bronchi and accompanying pulmonary arteries wascalculated from full inspiration and expiration HRCT of healthy men. Results : In 106 pairs of matched bronchi andpulmonary arteries, the brohchial-arterial ratios were 0.61$pm$0.18(upper lung), 0.72$pm$0.21(lower lung), and0.65$pm$0.20(total) at full inspiration and 0.51$pm$0.32(upper lung), 0.52$pm$0.15(lower lung), and 0.51$pm$0.26(total) atfull expiration HRCT, denoting a statistically significant increase in bronchial diameter of the entire lung atfull inspiration. The inner diameter of the bronchus was larger than the diameter of the accompanying pulmonaryartery by 0.1% at full expiration and 8.5% at full inspiration. Conclusion : The result of this study would behelpful in understanding the physiology of the lung during respiration.