Preoperative diagnosis of malignant tumors arising from mature cystic teratoma (MCT) of the ovary is not easy; malignant tumors are mostly diagnosed only postoperatively. Tumor size, serum tumor markers, and patient age have been proposed as risk factors for malignancy. This article reports a rare case of a giant, benign MCT of the ovary in a young woman (25 years old). It had a very large size (320 x 270 x 185 mm, 10 kg), a great number of teeth (> 300), and preoperative serum level of tumor markers were elevated (CA125, 875 U/mL(-1); CA19-9, 2087 U/mL(-1); CEA, 5.1 ng/mL(-1); AFP, 23.3 ng/mL(-1); SCC, 20.7 ng/mL(-1)). Based on clinical and laboratory data, tumor markers and tumor size when used alone or in combination do not appear to be useful in making a differential diagnosis between MCT and squamous cell carcinoma arising from MCT. However, radiologically detectable, well-differentiated teeth may be indicative of benignity.