Hemothorax is the presence of blood in the pleural space. The source of blood may be the chest wall, lung parenchyma, heart, or great vessels. Although some authors state that a hematocrit value of at least 50% is necessary to differentiate a hemothorax from a bloody pleural effusion, most do not agree on any specific distinction.
Hemothorax is usually a consequence of blunt or penetrating trauma. Much less commonly, it may be a complication of disease, may be iatrogenically induced,  or may develop spontaneously. 
Prompt identification and treatment of traumatic hemothorax is an essential part of the care of the injured patient. The upright chest radiograph is the ideal primary diagnostic study in the evaluation of hemothorax (see Workup). In cases of hemothorax unrelated to trauma, a careful investigation for the underlying source must be performed while treatment is provided.