Pleural effusion (PE) can change the equilibrium between volume of thoracic cavity and volume of intrathoracic structures, and it can disturb the function of respiratory system, heart, and diaphragm. PE alters scanty the pulmonary gas exchange, but it provokes restrictive changes in pulmonary function proportionally to fluid volume, increase thoracic diameters, and decrease lung compliance. PE can originate a syndrome similar to cardiac tamponade. Elevate intrapleural pressure increase intrapericardic pressure and this disturb the cardiac chambers filling, specially right chambers, and decrease cardiac output. Large PE can invert the diaphragm inducing paradoxical movement that origin a reduction of alveolar ventilation. All these alterations improve with drainage of the PE. We think that PE must always be drained, specially if respiratory failure is present.