Left hilar mediastinal lymph node enlargement is mostly seen in pneumonia, tuberculosis and lung cancer. 1. Pneumonia: acute and chronic pneumonia caused by disease-causing microorganisms, such as bacteria, viruses, mycoplasma, etc., will cause left hilar mediastinal lymph node enlargement when it spreads to mediastinal lymph nodes. 2. Tuberculosis: the specific infection of lung caused by tuberculosis bacillus, if improperly treated, the tuberculosis bacillus will invade the hilar lymph nodes, resulting in left hilar mediastinal lymph node enlargement. 3. Lung cancer: lung cancer can be metastasized through lymph, when lung cancer tumor is metastasized to left hilar mediastinal lymph node along lymphatic vessels, left hilar mediastinal lymph node enlargement will occur. There may be other reasons for left mediastinal lymph node enlargement, such as lymphoma. It is recommended to go to the hospital in time, improve the examination to clarify the cause of the disease, and then give targeted treatment or therapy.