The pathologic interactions between tumor and host immune cells within the tumor microenvironment create an immunosuppressive network that promotes tumor growth and protects the tumor from immune attack. In this study, we examined the contribution of the immunomodulatory enzyme indoleamine 2,3-dioxygenase (IDO) on this phenomenon. Expression of IDO was analyzed in colorectal cancer cell lines by reverse transcription-PCR and functional enzyme activity was assessed by high-pressure liquid chromatography. Semiquantitative immunohistochemistry was used to evaluate IDO expression in the tissue samples of 143 patients with colorectal carcinoma, and was then correlated with the number of tumor-infiltrating T cells and clinical variables. In vitro IDO expression and functional enzyme activity in colorectal cancer cells was found to be strictly dependent on IFN-gamma stimulation. Immunohistochemical scores revealed IDO-high expression in 56 of 143 (39.2%) tumor specimens, whereas 87 of 143 (60.8%) cases showed low IDO expression levels. IDO-high expression was associated with a significant reduction of CD3+ infiltrating T cells (46.02 +/- 7.25) as compared with tissue samples expressing low IDO (19.42 +/- 2.50; P = 0.0003). Furthermore, IDO-high immunoreactivity significantly correlated with the frequency of liver metastases (P = 0.003). Kaplan-Meier analysis showed the crossing of survival curves at 45 months. By multivariate Cox's analysis, IDO-high expression emerged as an independent prognostic variable (<45 months, P = 0.006; >45 months, P = 0.04). IDO-high expression by colorectal tumor cells enables certain cancer subsets to initially avoid immune attack and defeat the invasion of T cells via local tryptophan depletion and the production of proapoptotic tryptophan catabolites. Thus, IDO significantly contributes to disease progression and overall survival in patients with colorectal cancer.