Telomerase is a ribonucleoprotein enzyme that synthesizes telomeric DNA onto the ends of chromosomes, thereby preventing the replication-dependent shortening of these ends. Telomerase activity is detected in a wide range of cancers of various tissues, and its expression may be a critical step in tumor progression. Our objective was to determine if detection of telomerase activity may be an indicator for diagnosis of breast cancer and any association between telomerase activity and prognostic factors of breast cancer. Using a polymerase chain reaction-based telomerase activity assay, we examined telomerase activity in 30 breast cancer specimens (2 ductal carcinoma in situ, 28 invasive ductal carcinoma), 25 benign lesions (14 fibroadenomas, 11 fibrocystic diseases) and 24 normal breast tissues (13 adjacent to malignancy, 11 adjacent to benign lesion). Among surgically resected samples, telomerase activity was detected in 23 (77%) of 30 breast cancers. While telomerase activity was not detected in any of 11 specimens of fibrocystic disease and 11 adjacent normal tissues to benign lesion, surprisingly low levels of telomerase activity were detected in 5 (36%) of 14 fiboadenomas and 1 (7%) of 13 adjacent normal tissues to malignancy. There was no significant difference in expression of telomerase among prognostic factors of breast cancer. In summary, telomerase activity in breast cancer may be useful in diagnosis of breast cancer. We found no correlation between telomerase activity and stage, tumor size or LN status. Mechanisms of telomerase expression are still under investigation; therefore, the significance of telomerase expression in malignant tumors and their progression remains to be determined.