Clinical significance of determining the neopterin concentration in body fluids is reviewed. The results of researches on determining the neopterin concentrations in various infectious diseases (vector-borne diseases, herpes, respiratory and intestinal infections, as well as human immunodeficiency virus infection) conducted over the past 2 years are discussed. Neopterin is a biologically stable metabolite, which gives an advantage of its detection to assess the activity of the immune response. Previously neopterin was determined mainly by high-performance liquid chromatography. In recent years, enzyme-linked immunosorbent assay was introduced and frequently used for determining neopterin concentrations. It was shown that neopterin concentrations can vary also in the absence of the pathological process. In particular, some general factors such as race, age, body mass index, smoking and arterial pressure may influence on the concentrations of neopterin in the human body. Increased level of neopterin in body biological fluids and the kynurenine/tryptophan ratio are measured in diseases involving interferon-γ-mediated immune response activation. In this regard, the highest concentrations of neopterin and increased kynurenine/tryptophan ratio are observed in cases of infectious diseases, malignancies, transplant rejection, a number of cardiovascular and autoimmune diseases. It was shown that neopterin can be regarded as a highly specific marker of viral infection, and its blood concentration reflect the prognosis of the disease. Monitoring neopterin level may be useful to assess the severity and activity of an infectious disease, its clinical course, and to control the effectiveness of etiological treatment for many infectious diseases.