Abstract: We investigated whether oxidative stress, which contributes to aging, accelerates the telomere shortening in human cultured cells. The terminal restriction fragment (TRF) from WI-38 fibroblasts irradiated with UVA (365-nm light) decreased with increasing of the irradiation dose. Furthermore, UVA irradiation dose-dependently increased the formation of 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxodG) in both WI-38 fibroblasts and HL-60 cells. In order to clarify the mechanism of the acceleration of telomere shortening, we investigated site-specific DNA damage induced by UVA irradiation in the presence of endogenous photosensitizers using 32P 5′ end-labeled DNA fragments containing telomeric oligonucleotide (TTAGGG)4. UVA irradiation with riboflavin induced 8-oxodG formation in the DNA fragments containing telomeric sequence, and Fpg protein treatment led to chain cleavages at the central guanine of 5′-GGG-3′ in telomere sequence. Human 8-oxodG-DNA glycosylase introduces a chain break in a double-stranded oligonucleotide specifically at an 8-oxodG residue. The amount of 8-oxodG formation in DNA fragment containing telomere sequence [5′-CGC(TTAGGG)7CGC-3′] was approximately five times more than that in the DNA fragment containing nontelomere sequence [5′-CGC(TGTGAG)7CGC-3′]. Furthermore, H2O2 plus Cu(II) caused DNA damage, including 8-oxodG formation, specifically at the GGG sequence in the telomere sequence (5′-TTAGGG-3′). It is concluded that the formation of 8-oxodG at the GGG triplet in telomere sequence induced by oxidative stress could participate in acceleration of telomere shortening.