The main causes of pleural effusion due to rib fracture are as follows: 1. It may be due to bleeding inside the thoracic cavity at the time of the rib fracture, which may be caused by direct irritation or damage to the fracture end. It may also be due to a contusion of the lung at the same time as the rib fracture, which may result in bleeding in the chest cavity and subsequent effusion here. 2. This condition may be due to an inflammatory reaction to the rib fracture. After a rib fracture, inflammation in the thoracic cavity may occur, which may lead to inflammatory exudation in the pleural cavity, resulting in fluid accumulation. If the effusion is mild, the condition can be treated conservatively by waiting for its slow absorption. If the effusion is large enough to cause compressional atelectasis, closed drainage of the pleural cavity should be performed in such patients.