Insufficient immunosurveillance is an important aspect in early tumorigenesis and in the pathogenesis of malignant disease. In the later course of cancer, however, the development of immunodeficiency is considered the major reason for disease progression and death. In particular, immunodeficiency in cancer patients may develop as a long-term side-effect of the anti-proliferative and pro-apoptotic mechanisms elicited within the Th1-type immune response with an enhanced production of the pro-inflammatory cytokine IFN-γ being a key player. IFN-γ stimulates several anti-proliferative and thus tumoricidal biochemical pathways in various different cells including also tumor cell lines. These include inducible nitric oxide synthase, indoleamine (2,3)-dioxygenase, an enzyme degrading the essential amino acid tryptophan, and the production of reactive oxygen species and neopterin. Accelerated degradation of tryptophan and increased production of neopterin were found to parallel the course of malignant diseases. Moreover, a higher degree of these metabolic changes characterizes and predicts poor prognosis as well as has been demonstrated to be associated with the development of cachexia, anemia, and depression in terminally ill cancer patients.