Although many pulmonary and systemic diseases are known to cause pleural effusions, analysis of the pleural fluid pinpoints the cause in most cases. Distinguishing pleural transudates from exudates is an important step. Transudate effusions are caused by a small, well-defined group of illnesses (eg, cirrhosis, congestive heart failure). Exudative effusions, on the other hand, are associated with a wide variety of causes, including pneumonia, malignancy, TB, drug-induced reactions, and many others. Some effusions remain unexplained despite extensive tests. Surgical approaches may be appropriate for some of these patients, but the risks must be carefully weighed against the benefits.