During the reproductive period, human endometrium undergoes a pattern of cyclic change. Human endometrium undergoes a complex pattern of proliferation, secretory activity, and menstruation over an approximately 28 days period. Proliferative activity is highest during late proliferative phase under influence of estrogen, and minimal activity in the late secretory and menstrual phase. To study a possible change of telomerase activity during menstrual cycle, telomerase activities in normal and hormone treated endometrium were tested using telomerase repeat amplification protocol(TRAP) assay. Telomerase activities were detected in 9 of 10 proliferative endometrium(90%), and maximal activity was shown in late proliferative phase. Only 3 of 10 secretory endometrium(30%) revealed weak activity. However telomerase activity was not detected in menstrual phase endometrium(N 2) and senile endometrium(N=3). Four of tamoxifen treated endometrium(N 4) and 1 of provera treated endometrium(N 3) Levels of telomerase activity of treated endometrium(N 4) and late proliferative endometrium(N 6) were as high as them of detected in endometrial cancer and hyperplasia. Above findings reveal that telomerase activity of endometrium is changed according to menstrual cycle, And the level of telomerase activity is related to proliferative activity of endometrium that is dependent on the status of female sex steroid hormone and tamoxifen treatment.