YKL-40 is a secretory protein secreted among others by tumor cells and tumor-associated macrophages. Due to the structural homology this protein was classified to chitinases family CLP (chitinase - like protein) and to 18 of glycosyl hydrolase family, but it has no catalytic function. Elevated levels of YKL-40 in serum is observed in the inflammatory diseases of various aetiology and in cancers, such as breast, ovarian, colon or lung. The results of many studies suggest a significant relationship of YKL-40 with progression of cancer: incidence of metastases, shorter relapse-free survival and shorter overall survival. It is believed that YKL-40 may be a prognostic factor of cancer development and the patient's response to the applied therapy. Elevated levels of protein in serum of cancer patients may play a role in angiogenesis, proliferation and migration of tumor cells. Probably the mechanism of this phenomenon is the result of YKL-40 action by FAK and PI3K/AKT signaling pathways. Results obtained so far are largely based on an analysis of the YKL-40 level in the patients' serum and on the assessment of changes in the expression level of this protein in studies in vitro and in vivo.