Lung cancer is the most common type of cancer among males and females in developed countries. Currently, the five-year survival rate of lung cancer patients is 14%. Immune system has been considered to play an important role in the development and metastasis of lung cancer. In this study, natural killer (NK) cell infiltration was evaluated in the tumor tissues of 40 patients with stage I non-small cell lung cancer (NSCLC) to evaluate its effect on the prognosis. The evaluation included the tissue samples of 40 patients who underwent resection due to NSCLC between 1994 and 1998 in Ankara University Medical School, Chest Surgery Department. All the patients had stage I squamous cell carcinoma. The patients were followed-up for 4-8 years postoperatively. NK cell counts varied between 1 and 86 (mean: 13.63 +/- 11.86). The patients were divided into two groups according to their cell counts as low (n= 24) and high (n= 16). The mean survival time of the group with low NK cell count was 52 +/- 6 months and the mean five-year survival rate was 51%. The mean survival time of the group with high NK cell count was 58 +/- 4 months and the mean five-year survival rate was 68%. No statistically significant difference was detected between the two groups for five-year survival time (p= 0.15). As a result, in our study we did not find any significant effect of natural killer infiltration on tumoral tissue in prognosis of stage I NSCLC.