In patients with solid tumors, such as colorectal cancer, small cell lung cancer, melanoma, and ovarian cancer, high IDO expression is correlated with a poor prognosis and shorter overall survival.18, 19 In HCC, IDO was expressed in HCC cells following the stimulation of IFN‐γ,20 and our study confirmed that high IDO expression was a prognostic factor for poor survival for HCC patients. This evidence concerns the gene IDO1 and small cell lung carcinoma.