'Polypoid endometriosis' is a rare variant of endometriosis. We describe a case of an extremely large polypoid endometriosis mimicking a malignant tumor. A 37-year-old nulliparous woman was referred due to the rapid growth of an endometriotic cyst of the ovary and a high serum CA125 level. MRI revealed solid components in the pelvic mass. These preoperative clinical data were compatible with an ovarian carcinoma. A frozen section of the tumor biopsy showed as if an adenosarcoma, but finally the diagnosis of polypoid endometriosis with decidual change was made on permanent section. Polypoid endometriosis is a part of the differential diagnosis for malignant tumors in women with endometriosis, and we should consider carefully decision making for treatment.