Distribution of Leu-7+ cells in cancer tissues and regional lymph nodes was immunohistochemically examined to estimate the role of NK cells in colorectal cancer patients. Leu-7+ cells were rarely observed both in the primary and distant metastatic lesions, but the number of these cells was large in the germinal center of the lymph nodes. Studies of Leu-7+ cell population in the lymph node indicated that intermediate nodes in n0 and n1 groups showed significantly higher values than those in the control group. Comparative study of the population in identical patients revealed a significantly higher rate in the intermediate nodes than in the paracolic nodes both in n0 and n1 groups. These data suggest that Leu-7+ cells might be related to the defense mechanism of regional lymph nodes against tumor.