Herein, we report a case of giant lung hydatid cyst in a nine-year-old boy. For four years, he experienced mild chest pain and chronic nonproductive cough. He had a trauma resulted from a fall two days before admission. Chest X-ray showed misdiagnosed massive pleural effusion, and was aspirated in the other hospital. However, after admission, the computed tomography revealed a giant lung hydatid cyst filling the right hemithorax completely. Being considered by the diagnosis of ruptured lung hydatid cyst, he was treated surgically by right-thoracotomy. The existing hydatid cyst, (e.g., with a dimension of 30*22*20cm) filled all cavity of hemithorax extended from the right diaphragm to the apex of the lung situated in the right lower lobe. After evacuation of the hydatid fluid and laminated membrane, right lower lobectomy was carried out due to remaining no salvageable parenchyma without any complications. Also, the pathologic examination have confirmed hydatid cyst. In conclusion, giant hydatid cysts are probably misdiagnosed with massive pleural effusion in the endemic area. And, because of the risk of allergic reactions, anaphylactic shock and dissemination, it should not be aspirated.