BACKGROUND: Telomerase is an RNA-dependent DNA polymerase that compensates for the telomere shortening that occurs in its absence. Reactivation of telomerase is thought to be an important step in cellular immortalization, and recent studies have indicated that telomerase activity is often detected in primary human malignancies. The purpose of this study is to identify telomerase activity in breast cancer. MATERIALS & METHODS: Telomerase activities were analyzed in the samples of 12 breast cancer tissues and 11 benign breast disease tissues by TRAPeze ELISA detection kit (Oncor, Gaithersburg, USA). All samples were obtained from the excised mass at the time of specimen removal in the operating room and stored in liquid-nitrogen tank. RESULTS: Telomerase activity was detected in 10 of 12 (83.3%) breast cancer samples and 4 of 11 (36.4%) benign breast disease samples. The detection of telomerase activity in diagnosis of breast cancer has validity: 83.3% sensitivity rate, 63.6% specificity rate, 71.4% (+) predictability rate, 77.8% (-) predictability rate. The telomerase activity correlates with the estrogen receptor status (p=0.009). CONCLUSION: The telomerase activity can be detected in breast cancer sensitively. Further study with sufficient samples is needed to establish detection of telomerase activity as diagnostic tool in breast cancer.