Interlobular septa are 10-20nm long structures at the border of the secondary pulmonary lobules and which are usually perpendicular to the pleura in the periphery of the lung. Interlobular septa are composed of connective tisues and contain pulmonary venules and lymphatics. Diseases that affect one of the components of the interlobular septa may lead to thickeninig of the septa, which makes the visible on computed tomography (CT) scans. Some of the diseases that may be associated with this phenotypic feature are pulmonary edema, pulmonary lymphangitis carcinomatosis, leukemic or lymphomic involvement of the lung, diffuse alveolar hemorrhage, Nieman-Pick disease, and pneumonia. The terms septal lines or septal thickening may also be used to describe interlobular septal thickening. Earlier descriptions include the terms peripheral lines, short lines, and interlobular lines.