The secondary pulmonary lobule is a unit of lung supplied by three to five terminal bronchioles and contained by fibrous septa. High-resolution CT is able to show features of the secondary lobule, including interlobular septa, terminal bronchioles, and pulmonary arteries within a bronchiolovascular bundle. Because interstitial diseases have been shown to affect different components of the secondary lobule, high-resolution CT was used to compare the appearance of the normal secondary lobule with the abnormal secondary lobule in three patients with interstitial diseases primarily affecting lymphatic channels: lymphangitic carcinomatosis, sarcoidosis, and lymphangioleiomyomatosis. In lymphangitic carcinomatosis, the bronchiolovascular bundles and interlobular septa were thicker than those seen in a normal subject. In sarcoidosis, the interlobular septa and bronchiolovascular bundles were also thicker than normal. However, fibrosis caused distortion of the normal polygonal shape of the secondary lobule, not seen in either the normal subject or the patient with lymphangitic carcinomatosis. Lymphangioleiomyomatosis was characterized by multiple cysts within secondary lobules, often obliterating the normal polygonal appearance.