YKL-40 is produced by cancer cells and tumour-associated macrophages. YKL-40 may play a role in cancer cell proliferation, differentiation, survival, invasiveness, metastasis, in angiogenesis and the inflammation and remodelling of the extracellular matrix surrounding the tumour. Serum YKL-40 is a biomarker of prognosis, confirmed in 13 different types of cancer including > 2500 patients. Highest serum YKL-40 is found in patients with metastatic cancer with the shortest recurrence-free interval and shortest overall survival. Serum YKL-40 provides independent information compared with clinical characteristics and biomarkers, such as HER2, carcinoembryonic antigen, CA-125, prostate-specific antigen and lactate dehydrogenase. The authors hypothesise that inhibition of YKL-40 by monoclonal antibodies either directly or towards its receptor may be as efficient a cancer therapeutic as the monoclonal antibodies against HER2, HER1, vascular endothelial growth factor and CD20. Drugs inhibiting YKL-40 should be explored as new cancer therapeutics.