An aneurysmal bone cyst (ABC) is a benign but often rapidly expanding osteolytic multi-cystic osseous lesion that occurs as a primary, secondary, intra-osseous, extra-osseous, solid, or conventional lesion. A 15-year-old boy presented with a left-sided intrathoracic mass displacing the lung without bone destruction. The mass was totally resected without rib resection, and the pathological diagnosis was ABC. The clinical manifestations, etiology, management, and pathology are discussed, with a brief discussion regarding the difficulty in the preoperative differential diagnosis.