&NA;Thein‐vitrolymphocytotoxic responses of 27 preoperative colonic‐carcinoma patients and 61 healthy volunteers were correlated with age by linear regression analysis. Cytotoxicity obtained with leukocytes from the colonic‐cancer patients on the colonic cancer cell line decreased with increasing age. In contrast, cytotoxicity obtained with leukocytes from the control group on the same cell line did not correlate with age of the donor. Furthermore, the decline in cytolysis in the patient group could not be attributed to more extensive disease. These observations suggest that increasing age is an important factor associated with the decline in ability of colorectal cancer patients to generate a disease‐related cytolytic reaction.