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Outcome of TGFβ1 signaling is context dependent and differs between individuals due to germ-line genetic variation. To explore innate genetic variants that determine differential outcome of reduced TGFβ1 signaling, we dissected the modifier locus Tgfbm3, on mouse chromosome 12. On a NIH/OlaHsd genetic background, the Tgfbm3b(C57) haplotype suppresses prenatal lethality of Tgfb1(-/-) embryos and enhances nuclear accumulation of mothers against decapentaplegic homolog 2 (Smad2) in embryonic cells. Amino acid polymorphisms within a disintegrin and metalloprotease 17 (Adam17) can account, at least in part, for this Tgfbm3b effect. ADAM17 is known to down-regulate Smad2 signaling by shedding the extracellular domain of TGFβRI, and we show that the C57 variant is hypomorphic for down-regulation of Smad2/3-driven transcription. Genetic variation at Tgfbm3 or pharmacological inhibition of ADAM17, modulates postnatal circulating endothelial progenitor cell (CEPC) numbers via effects on TGFβRI activity. Because CEPC numbers correlate with angiogenic potential, this suggests that variant Adam17 is an innate modifier of adult angiogenesis, acting through TGFβR1. To determine whether human ADAM17 is also polymorphic and interacts with TGFβ signaling in human vascular disease, we investigated hereditary hemorrhagic telangiectasia (HHT), which is caused by mutations in TGFβ/bone morphogenetic protein receptor genes, ENG, encoding endoglin (HHT1), or ACVRL1 encoding ALK1 (HHT2), and considered a disease of excessive abnormal angiogenesis. HHT manifests highly variable incidence and severity of clinical features, ranging from small mucocutaneous telangiectases to life-threatening visceral and cerebral arteriovenous malformations (AVMs). We show that ADAM17 SNPs associate with the presence of pulmonary AVM in HHT1 but not HHT2, indicating genetic variation in ADAM17 can potentiate a TGFβ-regulated vascular disease.