Neopterin is a pyrazino-pyrimidine compound that belongs to the pteridine group. It is known to be a biochemical marker associated with cell-mediated immunity. It is produced by human monocytes/macrophages and dendritic cells from guanosine triphosphate (GTP) upon stimulation with interferon gamma (IFNγ), which is released by activated limphocytes Th. Neopterin is a very important clinic parameter, though the physiological role has not been exactly definited thus far. The level of neopterin reflects the stage of activation of the cellular immune system, which is important in the pathogenesis and progression of various diseases. Measuring its concentration in body fluids is used in many different areas of modern medicine, such as infectious disease, gastroenterology, transplantology and transfusiology, rheumatology or oncology. In neurological, cardio-vascular and autoimmune diseases, cell-mediated immunity is also activated, which is proved by the elevated level of this marker. Measurements of neopterin concentrations are also helpful in monitoring the therapy of patients infected with the HIV virus or treated by using immunomudulating therapy. As a result of measuring levels of neopterin in patients with neoplasms of digestive tract, increased concentration was proved, but it is not routinely used in everyday clinic practice.