The majority of the left lower lung basal segment mass foci are lung inflammatory exudation and interstitial edema caused by lung infection, tuberculosis and other diseases, and a small portion of them are lung malignant tumors and pulmonary fibrosis. 1. Lung infection and tuberculosis: virus, bacteria, mycoplasma, aspergillus, mycobacterium tuberculosis and other diseases can cause lung inflammatory exudation and interstitial edema, which can be manifested as mass foci of the left lower basal segment of the lungs on chest CT, and the inflammation and edema can be subsided after giving antibiotics, antiviral or antituberculosis treatment. 2. Lung malignant tumor: a small part of the left lower lung basal segment mass foci may be lung malignant tumor, the mass foci may exist for a long time, and later on, there may be changes such as diameter increase, density increase and lobular burr sign, which usually need surgical treatment. 3. Pulmonary fibrosis: pulmonary fibrosis is the end-stage pathological changes of fibroblast proliferation, extracellular matrix aggregation and lung tissue destruction after lung tissue injury. Common causes include autoimmune disease, radiotherapy injury, heredity, exposure to environmental hazardous substances and so on, and the imaging may be manifested as a mass of foci at the base of the lung. It is recommended to go to the hospital for further diagnosis and treatment, so as to clarify the cause of the disease and treat the symptoms and causes.