Pleural effusion that cannot be found is called unexplained pleural effusion, and generally three conditions need to be ruled out: first, pleural effusion caused by pneumonia, general chest CT examination can understand the inflammation of the lungs, and if the pleural effusion disappears after active anti-infection treatment, it is a pleural effusion caused by pneumonia. Second, pleural effusion caused by tuberculosis, also called tuberculous pleurisy, give pleural effusion examination, if diagnosed as tuberculosis caused by standardized anti-tuberculosis treatment, if the pleural effusion disappears can also be verified to belong to tuberculous pleural effusion. Thirdly, malignant pleural effusion, including those caused by lung cancer, can be further diagnosed through chest CT examination, fiberoptic bronchoscopy, pleural effusion and pathological examination, and if the pleural effusion is controlled or disappears with anti-tumor treatment, it can be verified as malignant pleural effusion. Pleural effusion other than these three is usually unexplained pleural effusion, which requires further improvement of other related system examination for further clarification.