This study was concerned with 2 issues: whether there are any normal intrathoracic anatomical structures that are relatively more difficult to identify on CRT (cathode-ray tube) monitor images, and if so, what their features are. Four physicians examined 7 intrathoracic anatomical structures on 50 chest roentgenograms that were displayed on CRT monitors and as conventional X-ray films on a light box. They categorized the visibility of each structure into 5 grades. The 7 structures were the spinal process behind the trachea, right margin of trachea, right main bronchus, left main bronchus, vascular shadow behind right diaphragm, vascular shadow behind cardiac shadow, and right margin of descending aorta. Luminescence ratios were calculated as the ratio of the density of a given viewpoint relative to adjacent areas on the images. We also examined the correlations between luminescence ratio and degree of difficulty in interpreting CRT images at a variety of selected luminescence ratios. Although the spinal process and trachea were easy to identify on CRT images as well as conventional films, other structures were difficult to recognize on CRT images. The luminescence ratio of each structure was higher on conventional film, and relatively low for those structures that were difficult to identify on CRT images. Each structure became gradually easier to identify with increasing luminescence ratios, up to a value of 1.07 maximum. However, this improvement in visibility leveled off beyond 1.07. We concluded that the luminescence ratio should be maintained at more than 1.07 to facilitate optimal identification of overlapping anatomical structures on CRT images.