A 6-year-old girl with hereditary multiple exostoses presented with spontaneous hemothorax and pericardial effusion. Chest computed tomographic scan showed left-sided costal exostoses protruding into the left side of the chest. She underwent successful thoracoscopic resection of 3 left rib exostoses and made a full recovery. This report illustrates this rare clinical scenario and reviews the previously published reports of this complication of costal exostoses.