The aim of this study was to seek for factors which lead to the early diagnosis of malignant transformation from mature cystic teratoma. Fourteen patients with malignant transformation from mature cystic teratoma of the ovary were analyzed retrospectively for precise clinicopathology and prognosis. The results demonstrated that although all the patients with stage Ia disease were disease-free, only 2 out of 7 patients were stage Ic to IV and disease-free in the follow-up period. Pre-operative imaging correctly diagnosed tumors as malignant in all stage Ic to IV cases, but only in 2 out of 4 stage Ia cases with magnetic resource and none of the 2 cases with computed tomography, respectively. In malignant cases, elevation of the serum SCC and CEA was observed in 90.9 and 88.9%, respectively. On the other hand, in benign cases, a false positive elevation of the serum SCC and CEA was observed in 23.5 and 14.3%, which turned out to be normal in 40 and 52.9% cases in the repeated study, respectively. In conclusion neither imaging analysis nor tumor markers including SCC and CEA accurately diagnose malignant transformation of mature cystic teratoma in its early stage, suggesting that a combination of diagnostic means is important. In the follow-up cases, repeated measurement of serum markers proved useful in ruling out false positive cases.