Telomerase, a ribonucleoprotein associated with synthesis of telomeric DNA, is postulated to play a role in cellular senescence and immortalization. Telomerase adds a hexonucleotide telomeric sequence to the chromosomal ends during replication and is preferentially expressed in most malignant and germ-line tissues but is usually undetectable in normal somatic cells. In the current study, 34 human endometrial tissues (20 malignant and 14 benign) were analyzed for telomerase activity by a nonradioactive PCR-based method using the TRAP-eze telomeric repeat amplification detection kit (Oncor). Nineteen of 20 (95%) endometrial carcinomas and 8 of 8 (100%) benign endometrial tissues from premenopausal women exhibited strong telomerase activity, whereas 6 of 6 (100%) benign endometrial tissues from postmenopausal women showed only weak telomerase activity. There was no correlation of telomerase activity with tumor grade, depth of invasion, or DNA content. Benign cycling endometrium, a rapidly proliferating tissue, features positive telomerase activity, although expression in nonneoplastic tissues has only rarely been previously reported. Only weak activity is detected in endometrial tissues after menopause, but telomerase activity can be strongly reactivated in patients who develop endometrial cancer.