The biosynthesis of the core 1 O-glycan (Galbeta1-3GalNAcalpha1-Ser/Thr, T antigen) is controlled by core 1 beta1-3-galactosyltransferase (T-synthase), which catalyzes the addition of Gal to GalNAcalpha1-Ser/Thr (Tn antigen). The T antigen is a precursor for extended and branched O-glycans of largely unknown function. We found that wild-type mice expressed the sialyl-T antigen (NeuAcalpha2-3Galbeta1-3GalNAcalpha1-Ser/Thr) primarily in endothelial, hematopoietic, and epithelial cells during development. Gene-targeted mice lacking T-synthase instead expressed the nonsialylated Tn antigen in these cells and developed brain hemorrhage that was uniformly fatal by embryonic day 14. T-synthase-deficient brains formed a chaotic microvascular network with distorted capillary lumens and defective association of endothelial cells with pericytes and extracellular matrix. These data reveal an unexpected requirement for core 1-derived O-glycans during angiogenesis.