The telomere, the terminal region of eukaryotic chromosomes, plays an important role in the stability of DNA replication and in the protection of chromosomal ends. To investigate whether the two-stage mechanism of cellular aging and immortalization in vitro is involved in the carcinogenesis and immortalization of human colorectal carcinomas, we examined for genetic alterations in the telomeres and in the p53, Rb, and K-ras genes. Based on our results, we discuss the effects that these genetic changes might have on mechanisms, such as the mortality stage 1 (M1), that normally prevent immortalization in carcinoma cells. Telomeric repeat-lengths (TRL) were measured by Southern blot analysis, and p53 Rb and K-ras gene variants were detected by PCR based assays. Thirty-six primary colorectal carcinomas were examined. Telomere alterations were found in 19 of 36 (52.8%) cases, while mutations of the p53 gene were observed in 15 of 36 (41.7%) cases and LOH involving p53 observed in 14 of 36 (38.9%) cases. Twelve of 15 (80%) cases with p53 mutations also showed altered TRL, so that p53 mutations were positively associated with TRL alterations (p=0.008). Ten of 14 (71.4%) cases with LOH of p53 also showed alteration in TRL, also revealing a positive association with TRL (p=0.09). The six cases with both p53 mutation and LOH all showed altered TRL. K-ras gene mutations and LOH involving the Rb gene were not associated with alterations in TRL. These results suggest that inactivation of p53 is one of the factors that promotes immortalization and overcomes M1 in colorectal carcinoma.