To evaluate the invasive potential of early tumorous lesions of the breast, especially DCIS, we have analyzed semiquantitative telomerase activity in well defined microdissected tissue areas from malignant or benign breast lesions from 145 patients. In order to prove the relationship to cell cycle defects, p53 and p21 proteins were analyzed in corresponding cryostat sections by immunohistochemistry. Telomerase activity was detected in 3 (33.3%) out of 9 benign breast lesions and 109 (80.1%) of 136 malignant tumors. Our study failed to demonstrate an association between telomerase activity, p21, established prognostic factors, such as lymph node status and metastasis, and clinical outcome. We found a high rate of cases (42.2%) with intratumoral heterogeneity concerning telomerase activity status. Telomerase heterogeneity was more obvious in samples with mixed tissue types, although we could not assign specifically the telomerase activity to lobular, ductal and intraductal tissue areas. Telomerase activity was detected in 81.8% of ductal carcinoma in situ (DCIS) which indicates telomerase reactivation as an early event in carcinogenesis. Fifteen (19.5%) out of 77 cases were positive for p53 protein in immunohistochemistry and found to be significantly more frequent in the subgroup with poor outcome. There was only a trend to an association of p53 with high level of telomerase. The prognostic relevance of telomerase activity for patients with benign breast lesions must be further investigated.