A pleural effusion with a bloody color is caused by bleeding from damage to blood vessels in the chest wall and lungs or leakage of red blood cells from pleural capillaries into the pleural cavity. It is commonly seen in chest trauma, pleural disease, lung disease, and other conditions such as liver disease, uremia, and left heart insufficiency. 1. Chest trauma: the broken ends of rib fractures may puncture the pleura of the chest wall and pulmonary vessels causing hemorrhage. Thoracentesis or closed drainage may damage intercostal blood vessels and cause bleeding. Chest surgical incisions and intrathoracic operations may also result in bleeding. 2. Pleural diseases: pleural mesothelioma, pleural metastasis, etc. can cause increased permeability of pleural capillaries, injury bleeding oozing or hemorrhage, and tuberculous pleurisy can also appear in the early stage of bloody pleural effusion. 3. Lung diseases: lung infection, lung cancer, pulmonary nodular disease, pulmonary embolism and other diseases can also appear bloody pleural effusion. 4. Other diseases: liver disease, uremia and left heart insufficiency can lead to increased permeability of pleural capillaries and leakage of red blood cells into the pleural cavity. Pleural effusion with blood color, it is recommended that the patient go to the hospital as soon as possible, so as not to delay the condition.