Osteochondromas also known as exostoses are usually asymptomatic. Hereditary multiple exostosis (HME) is a rare genelic disorder characterized by the formation ol several benign cartilaginous tumors from the perichondrium that flank the cartilage growth. It is most commonly detected as an incidental finding on X-ray or as a palpable mass. An 11-year-old boy presented sudden onset ol right-sided chest pain and dyspnea. A large amount of fluid effusion detected in the right half of the chest. Bloody mucus was removed by thoracocentesis. We performed a diagnostic thoracoscopy, where multiple osteochondromas of the ribs were visualized. Furthermore, a large irregular shape-edged bony spur was found arising from the posterior aspect of the anteriolateral portion ol the right seventh rib. Kulcsszavak: osteochondroma, genetic disorder, hereditary multiple exosfosls, thoracocentesis, thoracoscopy