frequency of apoptosis of tumorinfiltrating lymphocytes induced by fas counterattack in human colorectal carcinoma and its correlation with prognosis we investigated apoptosis in tumorinfiltrating lymphocytes tils obtained from 41 colorectal carcinomas by in situ nick translation isnt when the isnt labeling index li was determined as the number of positive nuclei per 1000 nuclei of til in tissue sections the median li was 120 range 230 the isnt li of colorectal carcinoma with lymph node metastasis was higher than that of colorectal carcinoma without metastasis the cases with a high li of 2120 had a significantly poorer prognosis than those with a low li we also confirmed immunohistochemically that a part of the tils expressed fas using the sections adjacent to what contained abundant isntpositive tils moreover fas ligand fasl expression was detected on the cell surface as well as the cytoplasm of colorectal cancer cells in 61 of cases apoptosis in tils was consistently seen more frequently in faslpositive cases than in faslnegative ones these findings indicate that the fasl expressed in colorectal carcinoma cells may kill the faspositive immune effective tils by means of a fasfasl system termed fas counterattack this tumor immune evasion induced by fasl may therefore affect the malignant potential of human colorectal carcinoma