To study the role of indoleamine 2,3-dioxygenase (IDO) in immune response and immune escape in gastric cancer, the human IDO gene was cloned by reverse transcription-polymerase chain reaction (RT-PCR) and inserted into the pIRES2-EGFP vector to construct the IDO expression vector (pIRES2-EGFP-IDO). BGC-823 cells were transfected with the vector by electroporation and selected stable expression with G418. IDO expression was determined by RT-PCR and Western blot analysis. The enzymatic activity of IDO was estimated by determining tryptophan and kynurenine concentrations in the cell culture medium by an amino acid analyzer. To assess the effect of IDO on T cell-mediated cytotoxicity and proliferation, T cells from patients with gastric cancer were co-cultured with the IDO-transfected BGC-823 cells in the presence or absence of 1-MT, a competitive inhibitor of IDO. Cells transfected with the vector expressed high levels of IDO mRNA and protein, and a significantly higher level of kynurenic acid was detected in the culture medium of the transfected cells compared to the non-transfected cells (P<0.001). T cells co-cultured with the IDO-transfected cells exhibited significantly lower cytotoxicity compared to the control group (P<0.05). Additionally, IDO-transfected cells treated with 1-MT exhibited higher toxicity compared to the untreated IDO-transfected cells (P<0.01). We conclude that IDO plays a key role in gastric cancer immune suppression, possibly by inhibiting T cell-mediated cytotoxicity and proliferation in vitro.