Dickkopf-3 (Dkk3) has been proposed as tumour suppressor gene and a marker for tumour blood vessels. We analysed the expression and function of Dkk3 in platelets and megakaryocytes from healthy controls and patients with BCR-ABL1-negative myeloproliferative neoplasms (MPN). Dkk3 protein and gene expression in platelets was compared with endothelial and other blood cell populations by ELISA, real-time PCR, and immunofluorescence. Moreover, megakaryocytes were isolated from bone marrow aspirates by CD61 microbeads. Immunohistochemical studies of Dkk3 expression were performed in essential thrombocythemia (ET), polycythemia vera (PV), primary myelofibrosis (PMF) and control reactive bone marrow cases (each n=10). Compared to all other blood cell populations platelets showed the highest concentration of Dkk3 protein (150 ± 19 ng/mg total protein). A strong DKK3 gene and protein expression was also observed in isolated megakaryocytes. Dkk3 co-localised with VEGF in α-granules of platelets and was released similar to VEGF upon stimulation. Addition of recombinant Dkk3 had no influence on blood coagulation (aPTT, INR) and platelet aggregation. Significantly more Dkk3+ megakaryocytes/mm2 could be found in bone marrow biopsies from patients with MPN (ET 40 ± 10, PV 31 ± 4, PMF 22 ± 3) than in controls (15 ± 3). The mean proportion of Dkk3+ megakaryocytes was increased in MPN as well (ET 83% ± 15%; PV 84% ± 12%; PMF 77% ± 8%) compared to controls (53% ± 11%). Dkk3+ megakaryocytes correlated with microvessel density in PV and PMF. We conclude that Dkk3 might be involved in the pathogenesis of MPN.