Telomerase, an RNA-containing enzyme, is associated with cellular immortality and malignancy. We investigated the role of telomerase during the multistage pathogenesis of breast cancer. We used the semiquantitative, PCR-based telomeric repeat amplification protocol assay for enzyme activity (42 specimens from 42 patients) and a radioactive in situ assay for expression of its RNA component (human telomerase RNA; hTR) for the identification of telomerase-positive cells in archival resection samples (n = 67 from 39 patients). Low telomerase activity was detected in 1 (14%) of 7 samples of benign breast disease, in 4 (67%) of 6 fibroadenomas, in 11 (92%) of 12 carcinoma in situ (CIS) lesions, and in 16 (94%) of 17 invasive breast cancers. There was a progressive increase in the mean telomerase levels with progressive increase in severity of histopathological change (P < 0.05). Almost all of 67 resection samples expressed hTR, irrespective of histology. Expression was low to moderate in some samples of normal epithelium and nonproliferative fibrocystic changes. hTR expression was limited to epithelial cells; expression in stromal cells, including those in fibroadenomas, was negative. Increased hTR expression was observed in some foci of apocrine metaplasia and atypical hyperplasia. Increased hTR expression was also observed in all CIS and invasive lesions, although considerable heterogeneity was noted. Focal up-regulation was frequently noted in CIS lesions in the vicinity of invasive tumors. Thus, up-regulation of hTR may be a predictive marker for invasive tumor development.