The differences between CT of pneumonia and lung cancer include the following aspects: 1. Pneumonia is an infectious disease of the lung and is clinically classified into lobar pneumonia, lobar pneumonia and interstitial pneumonia, which mainly manifests as lamellar, patchy or flocculent shadows in the lung, some of which are solid and can occur in any part of the lung or can be seen in one or both lungs. 2. Lung cancer includes central type lung cancer and peripheral type lung cancer, which also includes metastatic lung cancer. Central lung cancer is seen as a mass shadow near the lung door, which can be detected by CT. Peripheral type lung cancer is seen as isolated nodular shadow between the outer bands of lung, mostly accompanied by lobulated, short burr, irregular masses, with pleural traction sign, pleural depression sign, and even with pleural effusion. Therefore, the changes of pneumonia are not quite the same as those of lung cancer. Central type lung cancer can also cause obstructive pneumonia, and such lung infections are mostly caused by central type masses compressing bronchi, leading to pulmonary atelectasis and then producing lung infections. Therefore, fiberoptic bronchoscopy is needed to further clarify the cause of obstruction.