Indoleamine 2,3-dioxygenase 1 (IDO1), which is a key enzyme in tryptophan metabolism expressed in some subsets of normal and neoplastic cells, participates in tumor-induced tolerance. However, the mechanisms involved are not clearly understood. A hypothesis suggests that IDO1 may be involved in proliferation and conversion of regulatory T cells (Tregs). In this study, we evaluated the levels of IDO1 and forkhead box P3 (FoxP3) in non-Hodgkin lymphoma (NHL) tissues and performed ex vivo experiments to investigate the role of IDO1 on T-cell tolerance in NHL. The results showed that expressions of IDO1 mRNA and protein were coincidentally higher in NHL tissues than in reactive hyperplasia of lymph node tissues. Up-regulation of IDO1 was correlated with later clinical phases, larger tumors and higher serum lactate dehydrogenase (LDH), and indicated a worse prognosis. FoxP3 mRNA and protein levels were markedly increased alongside elevated IDO1 levels. Co-culture of murine CD4 + CD25- T cells with A20 cells could initiate the conversion of CD4 + CD25+ T cells, which showed a suppressive function in the mixed lymphocyte reaction. Moreover, the potent inhibitor of IDO1, 1-methyl-l-tryptophan, attenuated the conversion of CD4 + CD25- T cells into CD4 + CD25+ FoxP3 + T cells. The results suggested that up-regulation of IDO1 in NHL tissues could induce local immune tolerance by favoring development and infiltration of FoxP3 + Tregs through the conversion of CD4 + CD25- T cells into CD4 + CD25+ FoxP3 + T cells in the tumor microenvironment. This could be a novel mechanism of NHL escape from immune control.