During twenty one years from 1934 fo 1954, there were 1,974 cases treated by radiation therapy in the gynecological department of Okayama University. The authors have made statistical observations on the formation of thrombi and emboli, making a comparison with the result in the operated cases. Data obtained were as follows:-- The formation of thrombus; 11 cases (0.56%), in which 9 cases (0.46%) were thrombophlebitis. It was markedly less in number comparing with that in operative cases. The embolus was only one case (0.05%). The formation of thrombi showed a tendency to occur much frequently in patients over 41 years and in anemic cases. It was evident that the formation of thrombi occurred much in cachectic women and in patients who had retarded menarche. It was proved that the thrombophlebitis occurred much in cases complicated with intrapelvic inflammation and increased in number along with radiation processes. Every thrombophlebitic patient died of the recurrent cancer in the early course, and the permanent cure was very doubtful.