Whether or not a 4mm ground glass shadow in the upper lobe of the left lung is serious should be determined by the nature of the nodule. Benign nodules are usually not serious and malignant nodules are more serious. Benign left upper lobe ground glass shadow 4mm is often caused by bacterial, viral, tuberculosis and other infections, can be treated with levofloxacin, cefaclor, ribavirin, isoniazid and other medications, through the regular and effective drug treatment, the ground glass nodule shadow may shrink or disappear. On the other hand, malignant ground-glass shadow 4mm in the upper lobe of the left lung is mainly caused by early lung cancer or metastatic cancer, which is more serious and often grows rapidly, and some malignant nodules even show lymphatic and vascular invasion and metastasis to distant organs in the early stage, which affects the function of normal organs and tissues, and thus threatens the patient’s life safety. It is suggested that patients with 4mm ground-glass shadow in the upper lobe of left lung should go to the hospital in time, improve the examination, clarify the nature of ground-glass shadow and carry out standardized treatment according to the doctor’s instruction.