Background and Objective Immune tolerance plays the key role in the initiation,development and metastasis of the malignant disease.However,the underlging mechanisms have not been completely elucidated. The aim of this study is to investigate the expression of indoleamine-2,3-dioxygenase(IDO) in non-small cell lung cancer(NSCLC) and the distribution of regulatory T cell(Treg) in NSCLC tissue and the metastastic lymph node,and the possible relationships between them. Methods This study involved 62 NSCLC patients and 15 control patients with benign diseases that operated in the affiliated Xinqiao hospital and Southwest hospital of the Third Military Medical University. The expression of IDO in lung cancer cell, the corresponding normal lung tissues and the benign control tissues were detected by SP immunohistochemistry(IHC) and Fluorescent semi-quantitative RT-PCR.The distribution of Treg cell was detected by SP IHC too. The difference of positive rates of IDO was evaluated by Chi-square test and Fisher爷s exact test. Results The positive expression rates of IDO in lung cancer cells corresponding normal lung tissues and bengin disease tissues were 67.7% (42/62), 0 and 13.3% (2/15), respectively.There were significant differences between the former and the latter two (P 0.05). There was significant difference between the positive rate of IDO expression and the distribution of Treg cell(P