We evaluated patterns of abnormal lung parenchyma on CT scans in six specific chronic lung diseases and then applied those findings in the differential diagnosis of these lung parenchymal patterns in 56 subjects. There were 48 patients with chronic lung diseases (43 with histologic proof) consisting of usual interstitial pneumonia (n = 20), sarcoidosis (n = 16), lymphangitic carcinomatosis (n = 7), lymphangioleiomyomatosis (n = 2), drug toxicity (n = 2), and eosinophilic granuloma (n = 1). Including eight CT scans of normal control subjects, 56 CT scans were assessed independently by two readers (R1 and R2). Chest radiographs, most of which were obtained within 1 week of CT examination, were available in 48 of the 56 subjects. CT scans were evaluated for specific parenchymal features including disease distribution, lung distortion, thickening of bronchovascular bundles and polygon walls, bronchiectasis, cysts, and nodules, to determine the association of each abnormal feature with the different diseases. Diagnosis was then made from the overall CT appearance of the lungs and, on a separate occasion, from the appearance of the chest radiograph. The correct diagnosis was made from the CT appearance in 54 of 56 patients (R1) and in 50 of 56 patients (R2). Correct diagnoses were made from the chest radiographs in 42 of 48 patients (R1) and 43 of 48 patients (R2). We have identified features that are reproducible and useful when describing CT scans of patients with chronic lung diseases. Interpretation of the appearance of the lung on CT scans was accurate in diagnosing usual interstitial pneumonia, sarcoidosis, and lymphangitic carcinomatosis.