Neopterin is excreted at high levels during disease states challenging the T-lymphocyte-macrophage system. Stimulation of T-lymphocytes in mixed lymphocyte culture in the presence of macrophages is accompanied by raised neopterin levels in the supernatant. Neopterin may therefore be considered a specific indicator of cell-mediated immunity. Pretreatment analysis of patients with hemopoietic (191), urological (82) and gynecological (224) neoplasia revealed neopterin levels above the upper normal limits, with frequencies ranging from 57% (cervical carcinoma) to 96% (chronic lymphocytic leukemia). In patients with tumors of the genitourinary tract or with hemopoietic neoplasms, urinary neopterin levels were correlated with the tumor burden and became normal when remission was achieved. In gynecological cancer long term follow-up studies showed the value of neopterin excretion levels as a tumor marker. Tumor progression was indicated early by a rising neopterin level. Based on a comparison of different tumor types, the association of cell-mediated immunity and of neopterin excretion levels in less differentiated neoplasm is discussed.