Context. Malignant transformation in mature teratoma is a rare event, only 1-2% of cases: the most frequent histologies are squamous cell or papillary thyroid carcinomas: to the best of our knowledge, a papillary renal carcinoma was never been described before. Considering that to recognize a malignant transformation preoperatively is nearly impossible, it should be pointed out the utility of intraoperative consultation in those cases that present even a slighty alteration of some parameters. Patient. A 47-year-old woman went to the doctor for hemorrhage during menstruation. During the pelvic ultrasound examination, a semi-solid bilayer formation, partially fluid, of 42x32x36mm diameter, with increased vascularization was found in the left ovary. Tumor markers were normal except for a slightly alteration of CA125. Patient was programmed for laparoscopic surgery: the annex was totally removed and sent to analyze. Results. The histological examination revealed a mature cystic teratoma (MCT) with malignant transformation. The neoplasia (1,5cm) presented tubular-papillary architecture, severe atypia, mitotic count 5/10HPF, subepithelial deposits and foamy macrophages. Immunophenotype was CK7,CAM5.2,Ber-EP4,EMA,CD56,NSE positive, CK20,Tireoglobulin,TTF-1,CA19-9,S-100,GCDFP-15, calretinin,WT-1 negative, ER focally positive, PgR negative. Our diagnosis was papillary renal carcinoma Type 2 with neuroendocrine aspects in MCT. Outcome. The oncological commission decided there was not any indication to proceed with further procedures, but to continue with periodic follow-up. At 8 months after surgery, the patient is in a good state of health. Conclusion. In our opinion, the case is emblematic and unique, not only for the peculiarity of the histotype, never described before in MCT, but also as remark of the correct approach to the management of patients with lesions that can be turn out not to be benign after surgery. When the laparoscopic surgery is the selected procedure, it is mandatory to avoid every possible dissemination of the fluid content of the MCT into the intra-abdominal space. Moreover, the presence of altered parameters (as vascularization or tumoral markers) preoperatively, even slightly, could suggest an intraoperative consultation to determine if they are consistant with a malignant transformation.