© 2017 American Heart Association, Inc. Background and Purpose - Brain arteriovenous malformation (bAVM) is an important risk factor for intracranial hemorrhage. Current therapies are associated with high morbidities. Excessive vascular endothelial growth factor has been implicated in bAVM pathophysiology. Because soluble FLT1 binds to vascular endothelial growth factor with high affinity, we tested intravenous delivery of an adeno-associated viral vector serotype-9 expressing soluble FLT1 (AAV9-sFLT1) to alleviate the bAVM phenotype. Methods - Two mouse models were used. In model 1, bAVM was induced in R26CreER;Eng2f/2fmice through global Eng gene deletion and brain focal angiogenic stimulation; AAV2-sFLT02 (an AAV expressing a shorter form of sFLT1) was injected into the brain at the time of model induction, and AAV9-sFLT1, intravenously injected 8 weeks after. In model 2, SM22αCre;Eng2f/2fmice had a 90% occurrence of spontaneous bAVM at 5 weeks of age and 50% mortality at 6 weeks; AAV9-sFLT1 was intravenously delivered into 4- to 5-week-old mice. Tissue samples were collected 4 weeks after AAV9-sFLT1 delivery. Results - AAV2-sFLT02 inhibited bAVM formation, and AAV9-sFLT1 reduced abnormal vessels in model 1 (GFP versus sFLT1: 3.66±1.58/200 vessels versus 1.98±1.29, P<0.05). AAV9-sFLT1 reduced the occurrence of bAVM (GFP versus sFLT1: 100% versus 36%) and mortality (GFP versus sFLT1: 57% [12/22 mice] versus 24% [4/19 mice], P<0.05) in model 2. Kidney and liver function did not change significantly. Minor liver inflammation was found in 56% of AAV9-sFLT1-treated model 1 mice. Conclusions - By applying a regulated mechanism to restrict sFLT1 expression to bAVM, AAV9-sFLT1 can potentially be developed into a safer therapy to reduce the bAVM severity.