Lung cancer may cause hemoptysis, but hemoptysis is not necessarily caused by lung cancer. Lung cancer is a kind of malignant tumor of lung, which is usually rich in blood vessels. If lung cancer causes local necrosis and destruction of blood vessels in the process of growth, it may cause bleeding, and hemoptysis will be formed when the blood is coughed out of the body. Hemoptysis caused by lung cancer is mostly blood in sputum. However, if bleeding is caused by erosion of large blood vessels, it may also cause fatal hemoptysis, which is life-threatening. Hemoptysis can also be seen in many other diseases, including pneumonia, tuberculosis, bronchial dilatation, malformation of pulmonary blood vessels and so on. Therefore, when hemoptysis occurs, it is recommended that patients seek medical attention as soon as possible to identify the cause of the disease and treat it aggressively.