Spitting brown sputum is not necessarily a precursor of lung cancer, and a variety of respiratory diseases can present with the symptom of spitting brown sputum. Defining the cause of the disease requires a combination of other tests. Spitting brown sputum may be caused by old bleeding in sputum, which can be seen in many kinds of respiratory diseases, such as pneumonia, tuberculosis, bronchiectasis, lung cancer, etc. It is not necessarily a precursor of lung cancer. It is not necessarily a precursor of lung cancer. Early stage of lung cancer patients do not have specific symptoms, such as common cough, sputum, shortness of breath, lethargy, fatigue and so on. For high-risk groups who have been smoking for a long time and have a family history of lung cancer, they can undergo regular screening for lung cancer, such as low-dose spiral CT examination, tumor markers, sputum cytology examination and so on. If you have the symptom of spitting brown sputum, it is recommended to consult a doctor in time, improve the examination, clarify the specific cause of the disease and then target treatment.