Global DNA hypomethylation, in particular that of the gene promoter sequence in gene hypermethylation, is a well-known characteristic of human cancer. Subtelomeres are enriched CpG islands; methylation is believed to be a potential epigenetic regulator. However, regulation on the telomere length remains largely unknown. To demonstrate this correlation, four nasopharyngeal carcinoma cell lines (CNE, CNE1, CNE2, 5-8F) were treated for 72 hours with 0, 1, or 2.5 μM of the demethylating agent 5-aza-2'-deoxycytidine (5-aza-dC). Subtelomeric (D4Z4) level methylation was evaluated with a bisulfite assay, the human telomerase catalytic subunit (hTERT) expression was assayed by reverse transcription-polymerase chain reaction, the telomerase activity was detected using a telomeric repeat amplification protocol assay, and the telomere length was measured by Southern blot terminal restriction fragment analysis. There was significant demethylation following 5-aza-dC treatment, and a strongly repressed hTERT expression decreased the telomerase activity and remarkably shortened telomeres. Thus, partial subtelomeric methylation does not repress hTERT expression; conversely, demethylation may downregulate hTERT expression and shorten telomeres.