Hereditary hemorrhagic telangiectasia (HHT) is a dominantly inherited vascular disorder that is heterogeneous in terms of age of onset and clinical manifestations. Endoglin is the gene mutated in HHT1, which is associated with a higher prevalence of pulmonary arteriovenous malformations than HHT2, where ALK-1 is the mutated gene. Endoglin is constitutively expressed on endothelial cells and inducible on peripheral blood activated monocytes so that protein levels can be measured by metabolic labeling and immunoprecipitation. We report the analysis of umbilical vein endothelial cells in 28 newborns from 24 families with a clinical diagnosis of HHT. Reduced levels of endoglin were observed in umbilical vein endothelial cells in 15/28 subjects and in activated monocytes of all clinically affected relatives tested, suggesting that these individuals had HHT1. No mutant protein was expressed at the cell surface in any of these cases, and a transient intracellular species was seen in samples of only two families, supporting a haploinsufficiency model. Quantitative multiplex PCR fragment analysis was established for the endoglin gene and revealed six mutations that were confirmed by automated DNA sequencing. An additional 10 mutations were identified in newborns by sequencing all exons. Of the 16 mutations, 10 were novel, three had been independently identified in related families, and three were previously known. Our data confirm that endoglin levels correlate with the presence or absence of mutation in HHT1 families, allowing the early identification of affected newborns that should be screened clinically to avoid serious complications of this disorder, such as cerebral arteriovenous malformations.