Human telomeres, the nucleoprotein complexes at chromosome ends, consist of tandem arrays of TTAGGG repeats bound to specific proteins. In normal human cells, telomeres shorten with successive cell divisions^1,2, probably due to the terminal sequence loss that accompanies DNA replication. In tumours and immortalized cells, this decline is halted through the activation of telomerase^3–5, a reverse transcriptase that extends the telomeric TTAGGG-repeat arrays^6–7. Telomere length is stable in several immortal human-cell lines³, suggesting that a regulatory mechanism exists for limiting telomere elongation by telomerase. Here we show that the human telomeric-repeat binding factor TRF1 (ref. 8) is involved in this regulation. Long-term overexpression of TRF1 in the telomerase-positive tumour-cell line HT1080 resulted in a gradual and progressive telomere shortening. Conversely, telomere elongation was induced by expression of a dominant-negative TRF1 mutant that inhibited binding of endogenous TRF1 to telomeres. Our results identify TRF1 as a suppressor of telomere elongation and indicate that TRF1 is involved in the negative feedback mechanism that stabilizes telomere length. As TRF1 does not detectably affect the expression of telomerase, we propose that the binding of TRF1 controls telomere length in cis by inhibiting the action of telomerase at the ends of individual telomeres.