Small nodule 0.3cm in the upper lobe of the left lung is usually not serious. Small nodules in the upper lobe of the left lung are commonly caused by lung inflammation, tuberculosis, lung tumor, etc. If the lung tumor is malignant, it will be more serious. 1. Lung inflammation: If there are bacterial, viral, fungal infections in the upper lobe of the left lung, resulting in inflammation of the lung, small nodules will be produced. In this case, 0.3cm nodule is not serious, usually after infection control, it can gradually become smaller or even disappear. 2. Tuberculosis: 0.3cm nodules in the upper lobe of the left lung may also be related to tuberculosis, which can cause serious harm to the human body if it is not actively treated, and is relatively serious at this time. However, after taking active treatment measures, the condition can be effectively controlled and most of them can be cured. 3. Lung tumor: 0.3cm nodule in the upper lobe of the left lung may also be related to lung tumor; 0.3cm nodule is generally benign lesion, which needs regular review; the possibility of malignant tumor is extremely low, but it cannot be completely excluded; if the review finds that the nodule changes fast and the shape is irregular, the malignant tumor needs timely surgical treatment. It is recommended that the patient actively cooperate with the doctor to improve the relevant examination, clarify the diagnosis and take effective measures.