The ribonucleoprotein enzyme telomerase synthesizes telomeric DNA onto chromosome ends. Telomere length is maintained, by the presence of telomerase activity, in the vast majority of primary tumours and stem cells, suggesting that telomere maintenance is essential for cellular immortalization. Recently, the telomerase RNA component in human and mouse (TERC and Terc, respectively), a telomerase-associated protein TEP1/TLP1 (refs 6,7) and the human catalytic subunit protein TERT (refs 8,9) have been identified. To examine the role of telomerase in telomere maintenance and cellular viability, we established Terc-deficient embryonic stem (ES) cells. It is known that telomerase activity is absent in cells from Terc-knockout mice. Although the study showed that telomere shortening was observed in the Terc-deficient cells from first to six generation animals, whether telomerase-dependent telomere maintenance was essential for cellular viability remained to be elucidated. To address this issue, we examined Terc-deficient ES cells under long-term culture conditions. Accompanying the continual telomere shortening, the growth rate of Terc-deficient ES cells was gradually reduced after more than 300 divisions. An impaired growth rate was maintained to approximately 450 divisions, and then cell growth virtually stopped. These data clearly show that telomerase-dependent telomere maintenance is critical for the growth of mammalian cells.