The hypothesis that telomeres in colorectal cancer cells exhibit age-related shortening, as in normal cells of the colorectal epithelium, was tested with samples of non-cancerous mucosa and cancer tissue from 124 patients (aged 29-97 years). Shortening with aging could be demonstrated for both normal and cancer tissues; regression analysis showed rates for length reduction of 44 and 50 base pair/year, respectively. Straight, essentially parallel, lines were obtained for the two cases, normal tissue values being about 2 kilobase pairs (kbp) higher, with a significant correlation between data at the individual patient level.