The most common cause of hemothorax is chest trauma. Hemothorax can also occur in patients with lung or pleural cancer, or in patients with a defect of the blood clotting mechanism. The condition is also commonly linked with thoracic or heart surgery, and can also occur in patients who suffer pulmonary infarction (death of lung tissue).

In blunt chest trauma, a rib may lacerate lung tissue or an artery, causing blood to collect in the pleural space. In penetrating chest trauma, a weapon such as a knife or bullet lacerates the lung.

A large hemothorax is often a cause of shock in a trauma victim. Hemothorax may also be associated with pneumothorax (air trapped in the pleural cavity). Depending on the amount of blood or air in the pleural cavity, a collapsed lung can lead to respiratory and hemodynamic failure (tension pneumothorax).

Hemothorax can also be a complication of tuberculosis.