Analysis of endometrial carcinoma of 15 cases and the normal endometrium from 12 control cases was made using flow cytometry. The proliferetive activity and the ploidy level of the tumors were determined and a comparative study of the tumors classified by the nuclear grade of the tumor was made. In comparison with normal endometrium, endometrial carcinoma tissue had a significantly larger S-phase fraction of the cell cycle. Moreover, among the endometrial carcinoma cases, the percentage of S-phase cells was found to increase with increases in abnormalities of nuclear morphology. Endometrial carcinoma cases had higher levels of G2 +M phase fractions than normal endometrium, but not significantly so. The proliferation index of endometrial carcinomas was significantly higher than that in normal endometrium, and it was found that proliferative activity became more robust the more irregular the nuclear morphology. With regard to the ploidy level, all of the normal endometria and Nuclear Grade 1 or 2 endometrial carcinoma cases showed a DNA distribution between diploid and tetraploid. In contrast, Nuclear Grade 3 cases could be subdivided into two groups, one which had a DNA distribution between diploid and tetraploid with an extremely high proliferation index and the other which had an aneuploid DNA stem line. endometrial carcinoma ; flow cytometry ; proliferation index ; aneuploid DNA stem line In recent years, the incidence of endometrial carcinoma has been increasing worldwide (Lotocki et al. 1983) and in Japan as well (ref. 9, 1980). The increase emphasizes the need for improved understanding of the tumor characteristics and the related prognosis of such lesions. A close relationship between the histologic grade and prognosis of endometrial carcinoma has long been suggested (Lotocki et al. 1983) and it has also been reported that the cytologic nuclear grade of carcinomas may provide a still more accurate basis for prognosis than histologic grade (Christopherson et al. 1983).Due to recent advances in flow cytometry (FCM) using the LASER technique, it has become possible to use a cytochemical approach rapidly and efficiently at the cellular level in carcinoma cases. In the present study, we have