In this review, the role of kynurenine pathway enzymes (tryptophan 2,3-dioxygenase, indoleamine 2,3-dioxygenase, formamidase, kynurenine aminotransferase, kynurenine 3-hydroxylase, kynureninase, 3-hydroxyanthranilic acid oxygenase, picolinic carboxylase, quinolinic acid phosphoribosyltransferase) in the synthesis of tryptophan products degradation was described. Taking into account the importance of disorders, in which kynurenine metabolites may play pathogenic role. It was been postulated that kynurenine metabolites may be involved in pathogenesis of many diseases of clinical importance. Therefore evaluation of correlations between kynurenine metabolites and other variables, both clinical and biochemical, would be of interest. Attention was paid to the potential use of those biochemical parameter as biomarkers of many disease activity.