PURPOSE: To evaluate differences in regional density of normal lung, as seen on CT, according to respiration and gravity. MATERIALS AND METHODS: The subjects were 15 healthy volunteers, all non-smokers and without previous pulmonary disease. CT scans were obtained at three selected levels through the apex, middle and basal lung at the aortic arch, carina and just above the diaphragm, respectively at both full inspiration (FVC) and full expiration (RV). Within these regions of interest and at the three scanning levels, lung density was measured in the anterior, lateral, and posterior portions of the peripheral lung field. RESULTS: Attenuation of the anterior portion of the lung was lower than that of the posterior portion(p<0.005); average lung attenuation increase from the anterior to the posterior portion was significantly greater during full expiration than full inspiration (p<0.005), and was significantly greater at the base of the lung than at the apex (p<0.005 on expiration, p=0.006 on inspiration). Lung density during inspiration was lower than during expiration (p<0.005); average lung density increase from full inspiration to full expiration was significantly greater in the posterior portion than in the anterior (p<0.005). In the former, the average increase at the base of the lung was greater than at the apex (p=0.007), but in the latter, the average increase at the apex was greater than at thebase (p<0.005). CONCLUSION: In normal lung, respiration and gravity cause regional density changes, as seen on CT, and result in difference of lung attenuation between dependent and nondependent portions and between the apex, middle and base of the lung, according to inspiration and expiration.