Background: colorectal cancer is the third cancer cause of death in Spain. It is important to investigate new tumoral markers for early diagnosis, disease monitoring and prevention strategies. Telomeres protect the chromosome from degradation by nucleases and endto- end fusion. The progressive loss of the telomeric ends of chromosomes is an important mechanism in the timing of human cellular aging. Telomeric Repeat Factor 1 (TRF1) is a protein that binds at telomere ends. Purpose: to measure the concentrations of TRF1 and the relationships among telomere length, telomerase activity, and TRF1 levels in tumor and normal colorectal mucosa. Method: from normal and tumoral samples of 83 patients who underwent surgery for colorectal cancer we analyzed TRF1 protein concentration by Western Blot, telomerase activity, by the fluorescent- telomeric repeat amplification protocol assay and telomere length by Southern Blot. Results: high levels of TRF1 were observed in 68.7% of tumor samples, while the majority of normal samples (59%) showed negative or weak TRF1 concentrations. Among the tumor samples, telomere length was significantly associated with TRF1 protein levels (p = 0.023). Conclusions: a relationship was found between telomere length and TRF1 abundance protein in tumor samples, which means that TRF1 is an important factor in the tumor progression and maybe a diagnostic factor.