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Hydatid cysts caused by Echinococcus granulosus are most commonly seen in the liver and lungs, rarely in the spleen, kidney, bone, heart, muscles and other organs. Retroperitoneal localization is very rare. A 28-year-old male patient was referred to our clinic with a cystic lesion of 4.5x7.5x11 cm in size on the right hemithorax. Cystotomy was performed with frenotomy for subdiyaphragmatic lesion. The cyst cavity extending to the retroperitone was evacuated. The cyst cavity, which was purified with hypertonic NaCl, was filled with diaphragm muscle and omentum. Diaphragm was primary repaired. Albendazole treatment was started and the patient was discharged on the 4th postoperative day.