Mucinous bronchoalveolar carcinoma often coughs up white mucus sputum or white foamy sputum, but the color and nature of the sputum alone cannot determine whether it is lung cancer. To confirm whether it is lung cancer, a series of tests should be performed as follows. Firstly, if nodular or mass shadows are found in the lungs on chest X-ray or chest CT, lung cancer is highly suspected in chest imaging, and the next step is to perform bronchoscopy or lung aspiration to take biopsy and further clarify the pathological diagnosis. If the pathology suggests lung cancer, the diagnosis is basically clear. White foamy sputum can be seen in many diseases, such as lung infection, bronchitis, pneumonia, tuberculosis, etc. It can also be seen in some lung diseases such as interstitial lung disease, so sputum alone cannot determine whether it is lung cancer.