Telomeres are believed to stabilize chromosomes through several mechanisms that are dependent upon specific DNA-DNA and protein-DNA interactions. Telomeres are maintained by the enzyme telomerase. Telomerase activity, which is below detectable level in almost all types of diploid cells, is re-activated in most immortal and cancer cells. For this study, we designed peptide nucleic acid (PNA) oligonucleotides targeted to the telomeric G-rich strand, and tested their efficacy to reverse the immortality of transformed human fibroblasts. Anti-telomere PNAs, transfected into human fibroblasts along with a selectable marker, resulted in a significant reduction in colony size and elicited cell death by apoptosis. This PNA inhibitor does not inhibit telomerase activity in vitro, suggesting a distinct cellular mechanism from known PNA inhibitors. A combination of this class of PNA inhibitor with a PNA that does block telomerase activity resulted in nearly complete inhibition of colony growth, induction of apoptosis, and an apparent reduction in telomere length. Each effect was greater than that evoked by either agent alone, indicating enhanced efficacy for therapeutic approaches that target multiple, distinct mechanism of telomere maintenance.