We report a case of a spontaneous hemothorax in a 15-year-old girl because of costal exostosis. This possibly may have been provoked by a nontraumatic rupture of markedly dilated pleural vessels because of long-standing friction between the exostosis and the pleura. The authors conclude that exostosis of the rib is a rare cause of hemothorax in children and should be considered among possible etiologies in diagnosis.