Objectives: Natural killer (NK) cells are thought to serve as a first line of defense against tumor cells. The NK activities of peripheral blood mononuclear cells and pulmonary lymphocytes in patients with lung cancer were known to be decreased compared to those of healthy controls. The NK activity was reported to be restored after curative surgery, and decreased again in recurrent cases. We performed this study to investigate the change of the NK activity after curative surgery and to elucidate the relationship between the NK activity and clinical course of the patients. Methods: We studied the changes of the NK activities of peripheral blood mononuclear cells in 22 patients with lung cancer before and after curative surgery. Results are shown as % lysis of K562 cells at 50: 1 effector: target (E: T) ratio. Results: 1) The mean NK activity of peripheral blood mononuclear cells in patients with lung cancer (29.0±14.9%) was significantly lower than that of healthy controls (37.8±25.5%), and there was no significant difference according to cell type and stage. 2) The NK activity after curative surgery (40.7±20.7%) was significantly increased compared with that before surgery (29.0±14.9%) (p<0.05). 3) The NK activity after 1 week of surgery (31.1±9.6%) was not significantly increased compared with that of before surgery, but that was increased significantly after 1 month (46.9±21.1%), and was maintained high after 3 months (39.2±20.1%) in patients without recurrence (p<0.05). Conclusion: From our results, we concluded that NK activity of the patients with lung cancer is restored after curative surgery.