A critical event in tumor growth and progression is the upregulation of angiogenesis. Thus, targeting angiogenesis has become an attractive treatment modality in cancer medicine. Our study analyzed various solid tumor types for the expression of Dkk‐3, a cystein‐rich, N‐glycosylated secreted member of the dickkopf protein family that has been proposed as a tumor suppressor gene. Tissue microarrays of gliomas (n = 30), high‐grade non‐Hodgkin's lymphomas (NHL, n = 80), colorectal cancer (n = 35) and melanoma (n = 30) were immunohistochemically analyzed for Dkk‐3 and CD31 expression. Moreover, the effects of Dkk‐3 were studied in vitro in primary endothelial colony‐forming cells (ECFC) and in vivo in a mouse melanoma model. In comparison to normal tissue, the number of blood vessels expressing Dkk‐3 was increased in glioma, high‐grade NHL, melanoma and colorectal carcinoma. Confocal laser scanning microscopy revealed that Dkk‐3 vesicles localized also in Weibel Palade bodies. In vitro cell proliferation and migration of ECFC was not significantly affected by adenoviral overexpression or siRNA‐mediated downregulation of Dkk‐3. Interestingly, tube formation in matrigel decreased after downregulation of Dkk‐3 and increased after adenoviral overexpression. Stable overexpression of murine Dkk‐3 in B16F10 cells significantly increased microvessel density in the C57/BL6 melanoma model. Thus, we postulate a novel function of Dkk‐3 in endothelial cells as a differentiation factor involved in remodeling the tumor vasculature. © 2007 Wiley‐Liss, Inc.