PURPOSE: To evaluate the effect of respiration on the sizes of intrathoracic vasculature, and the trachea,and the main bronchus. MATERIALS AND METHODS: Seventeen volunteers (10males aged 20-39 years and 7 females aged20-39 years) underwent spiral CT, between the apex and lowest base of the lung, collimation was 10mm, pitch was 1,and images were obtained at breath hold forced end-inspiration and breath hold forced end-expiration. Crosssecional areas or diameters were measured in each respiration state at the aorta (ascending, descending, lowerthoracic) and great branches, the IVC (thoracic, abdominal), the SVC, pulmonary artery (right main, leftdescending) and the tracheobronchus (trachea, left upper bronchus). Changes in the size of vessels and airwaysbetween the respiration states were evaluated and compared between inspiration and expiration. RESULT: Duringbreath-hold forced end-inspiration CT, the ascending, descending, and lower thoracic aorta and itsbranches(brachiocephalic, left common carotid, left subclavian) as well as the thoracic IVC and SVC and the rightmain and left descending pulmonary arteries decreased in size: during breath-hold forced end-expiration CT, thesize of all these vessels increased. For the trachea, left upper lobe bronchus and abdominal IVC, the situationwas reversed. Statistically significant changes(p<0.05) were noted in the ascending aorta and descending aorta,the lower thoracic aorta, the thoracic and abdominal IVC, the SVC, the right main and left pulmonary arteries, andthe trachea. CONCLUSION: During respiration, changes in the size of the thoracic vasculature and airways isprobably due to changes in intrathoracic pressure. In the measurement and diagnosis of stenosis or dilatation inthe intrathoracic vesculature and airways, respiration states should therefore be considered.