This study was made to explore correlations of telomerase activity with a variety of biological factors in breast tumors. Telomerase activity was examined using the TRAP assay in 59 breast tumors including 45 breast cancers, eight fibroadenomas, four mastopathies and two phyllodes tumors which were surgically resected between 1996 and 1999. The correlations of telomerase activity with various biological factors such as DNA ploidy pattern and cell cycle phase were investigated. As far breast cancers, clinicopathological findings such as tumor diameter, axillary lymph nodes metastasis, clinical stage, histologic type, and hormone receptors were studied retrospectively in terms of correlation with telomerase. The telomerase activity was significantly higher in breast cancers than in benign tumors (P=0.0108), and correlated with Sphase of cell cycle phase. In breast cancers, the larger in size and the more advanced in clinical stage, telomerase activity was significantly higher, though there were no significant differences in histologic types. And in case of positive lymph nodes metastasis, telomerase activity tended to be higher. These results indicate a possibility that telomerase activity may be one of useful plognostic indicators.