Objective Indoleamine 2, 3-dioxygenase (IDO) is a rate-limiting enzyme in tryptophan catabolism that plays an important role in the induction of immune tolerance.Although its detailed mechanism of action remains uncertain, the immunosuppressive property of IDO is widely accepted and has been explored for its induction of peripheral tolerance, especially in the development of graft tolerance.IDO prevents graft-versus-host disease (GVHD) and improves graft survival by inducing immune tolerance.We conducted this review to emphasize the role of IDO in GVHD after human allogeneic hematopoietic stem cell transplantation (allo-HSCT).IDO activity may represent a potential biomarker for the diagnosis and evaluation of GVHD after allo-HSCT.We conducted this review to emphasize the role of IDO in GVHD after allo-HSCT.