The present study focused on two representative hemodialysis centers of Guangdong General Hospital (GGH) and Wuhua People's Hospital (WHH) and aimed to investigate (1) the circulating IDO status in uremia, (2) the capacity of PBMCs to metabolize Trp, (3) whether the Kyn/Trp ratio reflects in vivo peripheral IDO activity, and (4) the relationship between circulating IDO and immune response and infection. This evidence concerns the gene IDO1 and uremia.