The frequency of intraepithelial lymphocytes (IEL) was investigated in 255 colorectal lesions: in 18 metaplastic polyps, in 159 tubular, serrated and villous adenomas, in 28 incipient adenocarcinomas and in 50 overt adenocarcinomas. > or = 21 IELs were found in 13.6% (12/88) of the adenomas with high grade dysplasia (HGD) but in none of the 18 metaplastic polyps, the 71 adenomas with low grade dysplasia (LGD), or the 50 overt adenocarcinomas. CD3-T, MHCII and TIA1 cell antigens were positive in IELs from tubular adenomas, villous adenomas, incipient carcinomas and overt adenocarcinomas. In the normal colorectal mucosa and in metaplastic polyps, CD3-T was positive whereas MHCII and TIA 1 were not expressed. On the other hand, Fas cell antigen was expressed in the normal mucosa and in metaplastic polyps but it was downregulated in neoplastic lesions. The upregulation of CD3-T, MHC class II molecules, and TIA 1 in IELs of neoplastic colorectal lesions suggests that they are activated and cytotoxic; however, they undergo apoptosis due to the downregulation of the FAS molecules.