We utilized the Cre/lox recombination system to transiently express the catalytic subunit of telomerase (hTERT) in normal diploid foreskin fibroblasts (BJ cells). A retroviral construct containing an hTERT cDNA, flanked by loxP-sites was introduced into near senescent BJ cells (population doubling 85). At population doubling (PD) 92, which exceeds the typical life span of these cells, we excised the gene via Cre-mediated recombination. All clones lost telomerase activity and showed telomere shortening over an additional 50 PDs. Interestingly, the average telomere length in these cells became shorter than in untreated BJ cells at senescence, This may be due to hTERT preferentially elongating the shortest telomeres, leading to greater length uniformity, In summary, transient telomerase expression and only a very small average telomere elongation by hTERT resulted in a 50% increase in life span of human fibroblasts. This suggests a potentially safe use of hTERT in tissue engineering.