We now show that growth of several human RCC cell lines as well as RENCA cells incubated in the presence of IDO inhibitors, kynurenine, or its metabolites, is not altered in vitro, yet growth of RENCA tumors in immune-competent mice in vivo is attenuated when IFNα is administered concurrently with the competitive IDO1 and IDO2 inhibitor methyl-thiohydantoin-DL-tryptophan (MTH-trp). Here, IDO1 is linked to renal cell carcinoma.