Giant mature teratomas (>20 cm in diameters) sometimes may contain malignant immature elements or tissues. These small foci of immature tissues could be missed at frozen section resulting inadequate initial surgical intervention. There is no large series in the literature either showing the accuracy of frozen section on huge ovarian teratomas or concerning the adequate surgical approach at the initial surgery for giant immature teratomas diagnosed as mature cystic teratomas at frozen section in patients with elevated tumor markers. We presented a case with unilateral giant immature ovarian teratoma and peritoneal gliomatosis which diagnosed as mature cystic teratoma at frozen section. Although elevated tumor markers (CA-125, AFP, CA-19,9 CEA, and lactate dehydrogenase), ascites and peritoneal implants were suggested malignant disease, staging surgery was not performed due to the benign report of frozen section. Since the patients having peritoneal implants of mature glial tissue have an excellent prognosis regardless of the grade of the ovarian teratomas, it may be suggested that surgical staging at the initial operation may not be performed in patients with giant immature teratoma diagnosed as mature teratoma at frozen section despite to the elevated tumor markers, presence of ascites or mature peritoneal implants. seems that using it in infertile patients has beneficial effects.