Whether a solid nodule 12mmx9mm in the lower lobe of the left lung is serious or not depends on the nature of the nodule; benign nodules are not very serious, while malignant nodules are more serious. Solid nodule 12mmx9mm in the lower lobe of the left lung is usually not serious if it is an inflammatory nodule in the lung caused by bacterial infection, viral infection, fungal infection, tuberculosis and other factors. Patients may have no obvious clinical manifestations, or they may have fever, cough and other symptoms, which can be treated with amoxicillin, ribavirin, amphotericin B and other drugs as prescribed by the doctor and the condition will improve. The solid nodule 12mmx9mm in the lower lobe of the left lung due to lung malignant tumor is more serious, lung malignant tumor is a consumptive disease, and can not be completely cured, and there may be the risk of distant metastasis, the patient can die due to multiple organ failure. Therefore, patients with solid nodules 12mmx9mm in the lower lobe of the left lung should go to the hospital in time, complete the examination, clarify the nature of the nodule and follow the doctor’s instructions for further treatment.