Exostosis occurs in the ribs either sporadically or as a manifestation of a genetic disorder known as hereditary multiple exostoses. Costal exostosis may cause chest pain and, on rare occasions, result in hemothorax, pneumothorax, hemopneumothorax, and diaphragmatic or pericardial laceration. We report a case of pneumothorax in a 16-year-old boy with exostosis in the right seventh and eighth ribs. He previously received a diagnosis of multiple exostoses and underwent costal resection. The lung and pleura were injured because of long-term friction between the exostoses and the visceral pleura.To the best of our knowledge, this is only the fourth report of pneumothorax caused by costal exostosis. Moreover, all previously reported cases of pneumothorax or hemopneumothorax caused by costal exostosis, including the present case, involved lower lobe injury.