Objective: To report cases who were suspected to have mature cystic teratoma in the preoperative and intraoperative periods, but were found to have malignancy together with mature cystic teratoma in the final histopathological examination. Materials and Methods: The medical records of 148 cases were retrieved. The records were used to review the sociodemographic properties, histopathology, intraoperative tumor size, the surgical procedure, adjuvant therapy, and follow-up of the patients who were suspected to have mature cystic teratoma in the preoperative and intraoperative periods, but were found to have malignancy together with mature cystic teratoma in the frozen or final histopathological examination. Results: Of the patients, 8.2% were found to have malignancy arising in mature cystic teratoma. The median age of the patients was 32- (min: 15, max: 66) years-old and the tumor size was 12.1 (min: 4, max: 25) cm. Six patients were established to have an immature teratoma, three had a carcinoid tumor, one had a primitive neuro-extrodermal tumor (PNET), one had serous borderline tumor, and one had a borderline mucinous tumor. Conclusion: Although a mature cystic teratoma is benign, since it may involve different degrees of malignancy, intraoperative attitudes and procedures should follow the rules that apply to the management of a complicated adnexal mass and the possibility of a malignant transformation should be in kept in mind when informing the patient in the preoperative period.