Pleural effusion is the accumulation of fluid in the pleural cavity. The cause of pleural effusion in the elderly may be due to pre-existing underlying diseases such as pneumonia, rheumatoid arthritis, cirrhosis of the liver, nephrotic syndrome, congestive heart failure, and may also be due to lung cancer. Pleural effusion refers to the presence of excessive fluid in the pleural cavity, which is a clinical sign and usually does not exist in a specific population. Therefore, the presence of pleural effusion in the elderly indicates that they are suffering from other underlying diseases, such as pneumonia, tuberculosis, rheumatoid arthritis, systemic lupus erythematosus, etc., which will increase the permeability of the pleura. Secondly, it may also be due to the presence of diseases such as cirrhosis, nephrotic syndrome, acute glomerulonephritis, etc., which decrease the colloid osmotic pressure in the pleural capillaries. In addition to this, if one suffers from diseases such as congestive heart failure, constrictive pericarditis, etc., which increase the hydrostatic pressure within the pleural capillaries, pleural effusion can also result. Elderly people with pleural effusion also need to be alert to tumors, such as lung cancer, about 10% of lung cancer patients have different degrees of pleural effusion, suggesting that metastasis of the tumor obstructs lymphatic reflux in the pleura or lungs. When the elderly are examined for pleural effusion, relevant examinations should be perfected to clarify the specific cause of the disease, and active treatment should be carried out.