Cerebral cavernous malformations (CCMs) are a prevalent class of vascular anomalies characterized by thin-walled clusters of malformed blood vessels in the brain. Heritable forms are caused by mutations in CCM1, CCM2 and CCM3, but despite the importance of these factors in vascular biology, an understanding of their molecular and cellular functions remains elusive. Here we describe the characterization of a zebrafish embryonic model of CCM. Loss of ccm1 in zebrafish embryos leads to severe and progressive dilation of major vessels, despite normal endothelial cell fate and number. Vascular dilation in ccm1 mutants is accompanied by progressive spreading of endothelial cells and thinning of vessel walls despite ultrastructurally normal cell-cell contacts. Zebrafish ccm2 mutants display comparable vascular defects. Finally, we show that ccm1 function is cell autonomous, suggesting that it is endothelial cellular morphogenesis that is regulated by CCM proteins during development and pathogenesis.