In order to assess the in vivo function of integrins containing the β8 subunit, we have generated integrin β8-deficient mice. Ablation of β8 results in embryonic or perinatal lethality with profound defects in vascular development. Sixty-five percent of integrin β8-deficient embryos die at midgestation, with evidence of insufficient vascularization of the placenta and yolk sac. The remaining 35% die shortly after birth with extensive intracerebral hemorrhage. Examination of brain tissue from integrin β8-deficient embryos reveals abnormal vascular morphogenesis resulting in distended and leaky capillary vessels, as well as aberrant brain capillary patterning. In addition, endothelial cell hyperplasia is found in these mutant brains. Expression studies show that integrin β8 transcripts are localized in endodermal cells surrounding endothelium in the yolk sac and in periventricular cells of the neuroepithelium in the brain. We propose that integrin β8 is required for vascular morphogenesis by providing proper cues for capillary growth in both yolk sac and embryonic brain. This study thus identifies a molecule crucial for vascular patterning in embryonic yolk sac and brain.