Intraepithelial lymphocytes (IELs) and Intraepithelial apoptotic granules (AGs) are found in the vast majority of colorectal adenomas, less frequently in incipient carcinomas and occasionally in advanced colorectal carcinomas. In colorectal adenomas, the activated and cytotoxic IELs undergo apoptosis by a Fas-FasL mechanism. In advanced invasive carcinomas lacking IELs, that mechanism cannot be activated. On the other hand, the peritumoural lymphocytes which surround some advanced invasive carcinomas may abrogate to-be-metastatic tumor cells, as treated cancer patients with peritumoural lymphocytes have a better 5-years' survival than those without that peritumoural barrier. In colorectal adenomas the host reaction (IELs) dysplastic cells Fas-dependent confrontation seems to prevent rapidly proliferating adenomas from becoming rapidly invasive carcinomas, since that process takes 10 to 20 years to evolve.