High density shadow of the left lung refers to the shadow of increased density seen in imaging examinations such as chest X-ray or CT, which is commonly seen in lung inflammation, lung nodules, tumors and so on. 1. Inflammation of the lungs: such as lobar pneumonia, the imaging examination can see a large inflammatory infiltration shadow or solid shadow. Patients usually have symptoms such as cough, sputum and high fever. 2. Lung nodule or mass: high-density shadow in the left lung, round or round-like, which may be related to Mycobacterium tuberculosis infection, and the patient may have systemic symptoms such as low-grade fever, lethargy, fatigue, cough, hemoptysis, and so on. 3. Malignant tumor of lungs: high density shadow in left lung may be caused by malignant tumor of lungs, such as lung cancer, which can be detected by bronchial fiberoptic endoscopy with high density of biopsy and pathological examination to clarify the nature of cancer. High density shadow in the left lung can also be seen in other diseases, and it is recommended that patients go to the hospital in time, so as to avoid delaying the condition.