In an attempt to clarify the clinical characteristics of synchronous primary endometrial and ovarian cancer (SPC), we reviewed the clinicopathological features of 13 cases treated in the Department of Gynecology and Obstetrics at Kyoto University Hospital over the last 6 years and compared them with 186 cases of primary uterine corpus cancer (PCC) and 136 cases of primary ovarian cancer (POC). Comparisons were performed based on clinicopathological factors, including age, BMI, parity, complication of thrombosis and FIGO stage. For SPC patients, the mean age was 51.5 years; 6 (46%) were nulliparous, and 7 (53%) had complicated thrombosis. All had well-differentiated endometrial cancer and 12 (92%) had endometrioid cancer in the ovary. The mean age of the SPC patients was significantly lower than that of the PCC patients (51.5 vs. 58.9 years). Thrombosis occurred in the SPC patients at a significantly higher rate than in both the PCC and POC patients. When the incidence of endometriosis and the regularity of menstruation were compared between patients who developed SPC with those who develop PCC at a young age (under 45 years), the SPC patients exhibited a significantly higher rate of endometriosis (100 vs. 35%), whereas the PCC patients exhibited a higher rate of irregular menstruation (53 vs. 15%, p=0.05). As for thrombosis, the age and FIGO stage of thrombosis-positive patients were significantly higher than those of thrombosis-negative patients in PCC and POC, while in SPC patients there was no such difference. In conclusion, this study demonstrated the differences in clinical features between SPC and PCC, and also novel features of SPC, namely endometriosis and thrombosis, which are essential in the management of this disease.