The most common causes for undiagnosed transudative effusions are congestive heart failure and hepatic hydrothorax. Pleural fluid N terminal pro-brain natriuretic peptide levels higher than 1500 pg/mL are virtually diagnostic of congestive heart failure. The most common causes for undiagnosed exudative pleural effusions are malignancy, pulmonary embolism, and tuberculosis. Clinical characteristics of patients with a malignant pleural effusion are symptoms for more than 1 month, absence of fever, blood-tinged pleural fluid, and CT findings suggestive of malignancy. Thoracoscopy is useful to establish the diagnosis of malignancy and tuberculosis.