Mesenchymal cystic hamartomas are uncommon tumors originating from nodules of primitive mesenchymal cells, and their occurrence in the lung is extremely rare. We present a case of surgically resected mesenchymal cystic hamartoma of the lung in a 49-yearold woman who presented without any symptoms. Chest computed tomography showed a solitary cystic lesion in the right lower lobe. The patient underwent preoperative marking by 0.2 ml lipiodol 4 days before surgery. Thoracoscopic partial resection of the right lower lobe was performed. The histological picture and immunohistochemical profile were compatible with this tumor. © 2011 The Japanese Association for Thoracic Surgery.