Telomerase is a cellular endogenous reverse transcriptase thought to play an important role in the maintenance of telomere length. We investigated the effects of 3'-azido-2',3'-dideoxyguanosine (AZddG) and carbocyclic oxetanocin G (C.OXT-G), of which the triphosphate derivatives AZddGTP and C.OXT-GTP show potent telomerase inhibition, on telomere length of human HL60 cells in culture. Although AZddG caused more significant telomere shortening than C.OXT-G, only a slight decrease of cell growth rate was observed.