To explore the role of indoleamine-pyrrole 2,3-dioxygenase (IDO), an immunomodulatory enzyme, in renal cell carcinoma (RCC).A total of 40 patients diagnosed as RCC in the Second Xiangya Hospital were included in this study. All patients received nephrectomy. The histopathological features of samples were assessed semi-quantitatively. IDO mRNA level in RCC and non-RCC renal tissues was determined by real-time quantitative PCR (RT-qPCR). And the expression of IDO protein in endothelial cells was examined by immunohistochemistry; a Kaplan-Meier survival curves was calculated on the basis of IDO mRNA level.Level of IDO mRNA in RCC samples was significantly higher than that in tumor-free samples with P<0.001. Patients with high IDO expression had an significantly longer survival time than those with low IDO expression (P=0.01). There was a statistically significant inverse correlation between IDO and proliferation marker Ki67. Patients with high IDO level were of low Ki67 level, and vice versa (P<0.01).IDO might be a prognostic biomarker for patients with RCC.