Indoleamine 2, 3-dioxygenase (IDO) suppresses T-cell function at least in part by altering cell metabolism. Hypoxia-inducible factor-1 (HIF-1) increases upon T-cell activation and alters cell metabolism favoring their differentiation to effector cells. The effect of IDO on HIF-1α expression and activity was evaluated. For this purpose, mixed lymphocyte reaction (MLR) was performed using the IDO inhibitor 1-DL-methyl-tryptophan and the p53 inhibitor pifithrin-α. L-tryptophan degradation and cell proliferation were assessed by enzyme-linked immunosorbent assay, whereas the expression of proteins of interest by western blotting. IDO inhibited cell proliferation, and in MLR-derived T-cells increased HIF-1α and p53, whereas it decreased c-Myc. Inhibition of p53 abrogated IDO-induced HIF-1α upregulation. IDO increased the p53 transcriptional targets p21 and TP53-induced glycolysis and apoptosis regulator. The transcriptional targets of both HIF-1α and c-Myc, hexokinase II and lactate dehydrogenase-A were decreased by IDO. Phosphorylated pyruvate dehydrogenase remained unaffected indicating that pyruvate dehydrogenase kinase, a transcriptional target of HIF-1α, is not affected by IDO. In human alloreactive T-cells, IDO up-regulates HIF-1α, by inducing p53 overexpression. However, HIF-1α remains transcriptionally inactive.