Chest pain due to hemothorax is rare. However, the possibility of traumatic hemothorax must always be suspected. We report a case of a 15-year-old girl with hemothorax after erosion of the intercostal artery of the left seventh rib by a histological proved solitary costal hyperostosis. She presented with a 2-month history of intermittent left-sided chest pain and 3 episodes of acute chest pain followed by dizziness and loss of consciousness. The chest radiograph revealed a large left pleural effusion and a thin-section chest computed tomography scan that was performed after drainage of the hemothorax indicated a spicule projecting inward into the chest from the left seventh rib. After thoracotomy, a solitary costal hyperostosis causing a traumatic pseudoaneurysm of the intercostal artery was removed by partial resection of the left seventh rib, the intercostals vessels were ligated, and the pseudoaneurysm excised.