In this study, Natural Killer Cell Activity (NKCA) in the peripheral blood was tested by 125I-udR release assay in 34 cancer patients before and one, two, and four weeks after radical surgery. Results were compared with that obtained from normal blood donors. It was found that NKCA was significantly decreased in cancer patients especially in those with lymphatic node and remote metastasis. It was also found that NKCA underwent a transient, statistically significant depression immediatly after the surgery which may have some implication in cancer metastasis sometimes seen during this critical period. The authors believe that better understanding of the kinetics of perioperative NKCA could provide information necessary on how to protect NKCA and prevent cancer dissemination.