Pleural mouse (also known as pleural fibrin body or thoracolithiasis) refers to a calcified or non-calcified loose body in the pleural space. They can be single or multiple, and a pathognomonic feature is their mobile nature. Although the exact etiology is not known, they are theorized to represent sequelae of prior pleural effusion/hemothorax or necrosis of mediastinal/pericardial fat. They are rarely symptomatic, and most often detected incidentally.