Human cartilage glycoprotein 39 (HC gp-39) known as the YKL-40 protein or chondrex is secreted by a number of cells, mainly by activated chondrocytes. This process takes place in systemic or local articular inflammation. Biological function of HC gp-39 remains unknown. It is suggested that HC gp-39 is a remodeling factor of the connective tissue or a growth factor as well as acts as enthothelial cell migration factor. In this way, HC gp-39 contributes to angiogenesis, the phenomenon important for development of rheumatoid arthritis or metastating of malignant neoplasm. HC gp-39 is hypothesized to be a potential autoantigen in patients with rheumatoid arthritis. Increased serum level of HC gp-39 was found in patients with rheumatoid arthritis, osteoarthritis and patients with inflammatory bowel disease, hepatic cirrhosis and malignant neoplasms. It is believed that the following phenomena lead to enhanced the HC gp-39 synthesis and secretion: degradation and remodeling of the articular cartilage, inflammation with new vessel formation, fibrosis. Diagnostic application of HC gp-39 determination needs further studies. It is suggested that it may be an important marker of degradation of the cartilage as well as fibrosis.