Pleural fluid is normally formed in small amounts to lubricate the surfaces of the "pleura," which is the thin membrane that lines the chest cavity and surrounds the lungs. A "pleural effusion" is an abnormal collection of this fluid.

Two different types of effusions can develop. Transudative pleural effusions are usually caused by a disorder in the normal pressure present in the lung. Congestive heart failure is the most common cause of transudative effusion. Exudative effusions form as a result of inflammation (irritation and swelling) of the pleura, which is often caused by lung disease. Cancer, pneumonia, tuberculosis and other lung infections, drug reactions, collagen-vascular diseases, asbestosis, and sarcoidosis are some diseases that can cause exudative pleural effusions.

The cause and type of pleural effusion can usually be determined by taking a sample of the fluid by thoracentesis (removing fluid via a needle inserted between the ribs into the chest cavity, using a local anesthetic).