Malignant pleural effusion is caused by malignant tumors invading the pleura, and is mostly bloody (wash-like color), and if combined with an infection, it may have different appearances depending on the pathogen. Malignant pleural effusion is often caused by direct invasion or metastasis of lung cancer, breast cancer and lymphoma to the pleura, while other tumors including gastrointestinal and genitourinary tumors can also be caused by malignant mesothelioma of the pleura; it is common among middle-aged and old people over 45 years old, and there are symptoms such as dull pain in the chest, coughing up blood sputum and weight loss; the pleural effusion can be seen to be bloody (flesh-wash color), with a large amount of fluid and rapid growth. It is important to note that the color alone cannot determine the nature of pleural effusion, a clear diagnosis needs to go to the hospital for relevant tests, such as tumor markers, thoracentesis to extract pleural fluid for routine and cytological examination of pleural fluid.