Chronic myeloproliferative neoplasms (MPN), encompassing essential thrombocythaemia (ET), polycythaemia vera (PV) and myelofibrosis (PMF), are featured by a chronic inflammatory state which is pronounced in myelofibrosis The value of YKL-40 as a biomarker of disease burden has been demonstrated in several different diseases, including cancer, diabetes mellitus and cardiovascular diseases. A state of chronic inflammation is shared by them all, YKL-40 also being involved in the severity of chronic endothelial inflammation, which today is considered of crucial importance for the development of atherosclerosis. The MPNs being cancers with a heavy burden of cardiovascular diseases we hypothesised that circulating YKL-40 might reflect the inflammatory process and potentially serve as a novel disease marker. Using ELISA, we measured YKL-40 in 15 patients with ET, 16 patients with PV, 17 patients with PMF and 30 healthy controls. YKL-40 was significantly elevated in PMF vs. control subjects, PMF levels median 43 ng/mL vs. controls median 28 ng/mL, P = 0.033. An increase from ET over PV may reflect the integrated impact of disease processes in MPNs.