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Telomeres are the specialized structures at the end of eukaryotic chromosomes. In humans and all other vertebrates, telomeric DNA consists of tandem repeats of the highly conserved G-rich sequence TTAGGG. 1–3 These repeats are thought to protect genomic DNA from degradation and deleterious recombination events. 1,2 Normal human somatic cells lose 50 to 200 bp of terminal telomeric DNA with each round of replication 4–6 because of the inability of DNA polymerase to replicate fully the ends of a linear DNA template 6–8 and presumably because of the lack of telomerase activity. 9,10 Telomerase is a ribonucleoprotein enzyme that uses its own integral RNA as a template for synthesis on the TTAGGG telomeric repeats to compensate for the normal loss of telomeric repeats during cell divisions. 11 Recently, a highly sensitive polymerase chain reaction (PCR)-based telomerase assay called the telomeric amplification protocol (TRAP) was developed to detect telomerase activity. 10,12 Using this method, telomerase activity has been found in various tissues. In cultured cells, 98% of immortal and no mortal cell populations were found to express telomerase activity. 10In vivo, telomerase activity appears to be repressed in somatic cells and tissue, except for the fact that some reproductive somatic cells revealed a low level of telomerase activity, such as epidermal cells, 13 intestinal mucosal cells, 14 peripheral blood monocytes, and hematopoietic progenitor cells. 15 In tumor tissue, telomerase activity was detected in 94% of neuroblastoma, 16 93% of colorectal cancer, 17 85% of gastric cancer, 18 and 85% of hepatocellular carcinoma specimens. 19 In lung cancer, telomerase activity was detected in almost all small cell lung cancer specimens and in approximately 80% of non-small cell lung cancer (NSCLC) specimens. 20 There has been no previous report regarding the telomerase activity and patient prognosis in NSCLC. In this study, we examined the clinical significance of the telomerase activity, especially regarding its prognostic impact in NSCLC.