Malignant gliomas are the most common type of primary malignant brain tumours, characterized by extreme proliferation and aggressive invasion. There is evidence for over-expression of the YKL40 gene in high-grade gliomas. The high serum levels of the glycoprotein are associated with poor prognosis of various inflammatory and tumour processes. We investigated the YKL40 mRNA level and protein expression in the tumour site and in the serum of high-grade glioma patients. The YKL-40 expression in 36 patients with glial tumours (astrocytoma grade III, glioblastoma) and 33 age-matched healthy persons was measured by gene analysis, immunohistochemistry and ELISA. YKL-40 serum levels in high-grade glioma patients compared to healthy subjects were significantly increased (P ≤ 0.05). A wide range of variability in YKL40 mRNA expression was found. YKL-40 staining in situ was more abundant in glioblastoma tissue than in anaplastic astrocytoma, with the lowest level in normal brain tissue. Our gene analysis revealed that in general, YKL40 mRNA in glioma patients was over-expressed versus normal brain. A significant correlation between YKL40 transcript and protein levels was observed (P ≤ 0.05). It could be speculated that the YKL-40 protein might contribute to glioblastomas' specific biological characteristics that distinguish them from grade III gliomas. A complex investigation of YKL40 expression was performed at the molecular and cellular levels in human high-grade gliomas. Serum YKL-40 concentrations increased with tumour grade and correlated positively with transcript rate, being the highest in glioblastoma. We provide evidence for a relationship between YKL40 expression and the malignancy of glial tumours.