Thoracentesis is a valuable diagnostic procedure in a patient with pleural effusion of unknown causation. Analysis of the pleural fluid will allow its categorization as either a transudate (a product of unbalanced hydrostatic forces) or an exudate (a product of increased capillary permeability or lymphatic obstruction) (Table 1). If the effusion seems to have an obvious source (e.g., in an afebrile patient with congestive heart failure and bilateral pleural effusions), diagnostic thoracentesis may be deferred while the underlying process is treated. The need for the procedure should be reconsidered if there is no appropriate response to therapy.1 Thoracentesis, as a therapeutic procedure, may dramatically reduce respiratory distress in patients presenting with large effusions.