Human uterine endometrium undergoes a complex pattern of changes in proliferation and secretory activity during the menstrual cycle. In the present study, telomerase activity in normal endometrium was examined using a non-radioisotope PCR-based telomeric repeat amplification protocol assay. Various levels of telomerase activity were detected in the 60 normal endometrial samples examined, depending on the phase of the menstrual cycle. Of 21 proliferative-phase endometrial samples, 20 (95%) expressed telomerase activity, whereas 8 of 19 (42%) secretory-phase or menstrual endometrial samples did (P = 0.002). Five of nine (56%) samples from atrophic endometrium from postmenopausal women also expressed telomerase activity. Eleven of 21 (52%) endometrial samples in the proliferative phase expressed high telomerase activity detectable after 100-fold dilution of extracts, whereas none of the 19 endometrial samples from the secretory phase or during menstruation and none of the 9 postmenopausal endometrial samples did (P < 0.001). The highest activity was observed in the late proliferative phase, but activity dramatically decreased with the progression of the secretory phase. Surprisingly, the levels of telomerase activity detected in the late proliferative phase were comparable to those detected in the endometrial cancers examined. Immunohistochemical analysis of the expression of proliferating cell nuclear antigen revealed that telomerase activity is closely correlated with endometrial cell proliferative activity. These findings indicate that normal endometrium expresses telomerase, the activity of which changes dramatically over the course of the menstrual cycle, suggesting in turn that telomerase is a regulated enzyme linked to cellular proliferation and that hormone functions may be involved in its regulation.