Cerebral cavernous malformations (CCMs) are common inherited and sporadic vascular malformations that cause strokes and seizures in younger individuals(1). CCMs arise from endothelial cell loss of KRIT1, CCM2 or PDCD10, non-homologous proteins that form an adaptor complex(2). How disruption of the CCM complex results in disease remains controversial, with numerous signalling pathways (including Rho(3,4), SMAD(5) and Wnt/beta-catenin(6)) and processes such as endothelial-mesenchymal transition (EndMT)(5) proposed to have causal roles. CCM2 binds to MEKK3 (refs 7-11), and we have recently shown that CCM complex regulation of MEKK3 is essential during vertebrate heart development(12). Here we investigate this mechanism in CCM disease pathogenesis. Using a neonatal mouse model of CCM disease, we show that expression of the MEKK3 target genes Klf2 and Klf4, as well as Rho and ADAMTS protease activity, are increased in the endothelial cells of early CCM lesions. By contrast, we find no evidence of EndMT or increased SMAD or Wnt signalling during early CCM formation. Endothelial-specific loss of Map3k3 (also known as Mekk3), Klf2 or Klf4 markedly prevents lesion formation, reverses the increase in Rho activity, and rescues lethality. Consistent with these findings in mice, we show that endothelial expression of KLF2 and KLF4 is increased in human familial and sporadic CCM lesions, and that a disease-causing human CCM2 mutation abrogates the MEKK3 interaction without affecting CCM complex formation. These studies identify gain of MEKK3 signalling and KLF2/4 function as causal mechanisms for CCM pathogenesis that may be targeted to develop new CCM therapeutics.