A 21-year-old male was admitted to our hospital complaining of chest pain. He had undergone operations on multiple exostoses of his lower extremities when he was thirteen and fifteen years old. Family history revealed multiple exostoses in his mother and one of his cousins. Chest roentgenograms showed right pneumothorax and a mass arising from the seventh rib in the right lower lung filed. Chest CT revealed that the mass was located in thorax but outside the lung. These findings suggested that the pneumothorax was secondary to the injury of the right lung caused by an exostosis arising from the rib. Upon operation, we found a laceration of the lower lobe and a hard mass the size of a pea protruding from the seventh rib. A small bony spicule was also found to be projecting from the sixth rib. The hard mass and the spicule were resected with a normal portion of the seventh and the sixth ribs. The pathological findings of these bony lesion proved to be consistent with exostosis. The patient's postoperative course was uneventful. This case of pneumothorax caused by an exostosis lacerating the lung is rare.