Many cancer and immortal cells exhibit telomerase activity that stabilizes telomere lengths and may be involved in cell immortality and carcinogenesis. Downregulation of telomerase has been reported during differentiation of hematopoietic, melanoma, glioma, and myelocytic leukemia cells. Moreover, normal human mammary epithelial cells immortalized by a p53 mutant have been reported to exhibit activation of telomerase. However, no information is available about the activity of telomerase during p53-mediated apoptosis of immortalized cells. We investigated the activity of telomerase during p53-induced apoptosis of the immortalized endothelial cell line ECV-304. ECV-304 cells were induced into apoptosis by infection with a recombinant adenovirus that facilitated expression of high levels of wild-type p53. Telomerase activity was measured by a PCR-based telomeric repeat amplification protocol (TRAP). Telomerase activity was found to be unaffected by overexpression of p53 and apoptosis in immortalized endothelial cells.