Abstract： Objective To analyze the clinicopathologic characteristics,treatment and prognostic factors in malignant transformation of mature cystic teratoma(MCT)of ovary.Methods The clinical data of 44 patients with MCT from January 1961 to June 2009 were reviewed.Results The median age of the 44 patients was 48 years(range,16-84 years).Mean tumor size was(16 ±6)cm.Thirty-two cases were diagnosed squamous cell carcinoma(73％,32/44),and 5 of them with the elevated level of serumal squamous cell antigen(SCC-Ag).Three of 37 cases(8％,3/37)were identified with malignant transformation in image examinations.Rapid frozen section examination and multiple-location biopsy were performed in 8 cases,and 5 of them were detected with malignant diseases.Twenty-two patients with disease confined within the unilateral ovary(10 with intact capsule,and 12 with ruptured capsule).Diseases extended extra ovaries in the others 22 patients.The median cumulative overall survivals were 126 and 10 months,respectively.The difference between the two groups was significant(P ＜ 0.01).Twenty-seven patients had no residual tumor after primary surgery.The median cumulative overall survivals between the patients with and without residual tumor were 10 and 84 months respectively,and there were significant difference between two groups(P ＜ 0.01).Seven selected patients with malignant disease confined within unilateral ovary underwent fertility-sparing surgery,and 2 cases of them had successful pregnancies and delivery,while other 4 cases with ruptured capsule recurred.Conclusions The most common pathology type of malignant transformation in mature cystic teratoma of the ovary is squamous cell carcinoma.Comprehensive pre-operation image examination and tumor marker level detection might be of great help in diagnosis.Tumor extension extraovary and residual tumor after surgery are the most significant poor prognostic factors.Early stage patient with ruptured capsule should be very discreet to choose fertility-sparing surgery.