Milled glass nodules measuring 3.9 mm with clear borders are usually associated with pneumonia, tuberculosis and other diseases and are generally not serious. However, the possibility of continued progression of malignant changes cannot be ruled out. 1. Pneumonia: Various pathogens such as bacteria, viruses, mycoplasma, chlamydia, etc. infect the lung tissue and cause pneumonia, which may be detected in the lung imaging examination such as chest CT with a size of 3.9mm and clear boundaries of the ground-glass nodule. These diseases are generally not serious and can be gradually recovered and cured through timely treatment. 2. Tuberculosis: It is mainly caused by infection with Mycobacterium tuberculosis and can be cured after regular anti-tuberculosis treatment. Because of the destruction and proliferation of lung tissues, there will be 3.9mm in size, clear boundaries of the ground-glass nodules, which are generally not serious. 3. Progression of malignant changes: If the size of 3.9mm, the border of the ground-glass nodule is clear, and in the course of follow-up observation as prescribed by the doctor, it is found that the nodule gradually increases in size, the border is blurred, and there are signs of lobulation and burr in the morphology, then it is considered that the nodule has progressed to malignant changes, which is more serious at this time, and it is necessary to remove it by surgery as soon as possible. In the case of ground-glass nodules with clear borders of 3.9mm in size, it is recommended to consult a doctor in time, have a regular review, and follow the doctor’s instructions for treatment.