YKL-40 is detected in the early nineties not only a glycoprotein secreted by cancer cells, but also of neutrophils, chondrocytes and endothelial cells. Biological function of YKL-40 are not exactly known, it is suggested that this protein participates in many physiological and pathological processes such as proliferation, angiogenesis, mitogenesis and remodelling. It is also a factor antyapoptotic and growth factor for some cells. Increased levels of YKL-40 in serum have been described in many diseases running with inflammation such as rheumatoid arthritis, systemic lupus erythematosus and Crohn's disease, and also in the group of cardiovascular diseases. Determination of the concentration of serum YKL-40 was also used in oncology. The tumors at various sites were found elevated levels of this marker in serum, as well as overexpression in tumor tissue. It was observed that higher titers YKL-40 levels are associated with shorter overall survival and disease-free period. It is suggested that measuring the concentration of YKL-40 in serum and its expression in tumor tissues may serve as a valuable and independent prognostic factor.