A 64-year-old woman without any respiratory symptoms was diagnosed as having pulmonary alveolar microlithiasis from the characteristic finding of her chest radiograms. Diffuse fine nodular shadows of microliths were seen throughout both lungs. Linear shadows, which were considered to be typical Kerley's lines (A, B, and C), were also noted. Conventional tomography and high resolution, thin-slice CT disclosed the anatomical structures and the distribution of interlobular septa in vivo, which were responsible for Kerley's lines. Lateral chest radiograms revealed oblique short linear shadows just behind the sternum, which were caused by interlobular septa at the mediastinal side of the upper lung. These shadows seemed to represent new septal lines.