In HHT, focal dilations of the postcapillary venules (indicated on images by V) progressively encompass the normal capillaries and establish a direct connection with dilated arterioles (indicated by A), leading to AVM formation, muscularization of large vessels, and dilation of capillary beds, which are often surrounded by inflammatory cells. At the microvascular level, AVMs appear as telangiectases on specific mucocutaneous areas (finger pads, lips, tongue, nasal and digestive mucosa). They are responsible for spontaneous and recurrent epistaxis/bleeding, leading to bleeding iron-deficiency anemia. At the macrovascular level, large AVMs mainly affect the liver, leading to high-output cardiac failure, and more rarely biliary ischemia and portal hypertension; lung AVMs can provoke ischemic stroke and cerebral abscess and, more rarely, hypoxemia and hemoptysis. Central nervous system AVM is rarely complicated by hemorrhage. Skin and liver sections were stained for H&E. Photo of the liver section was provided by J.Y. Scoazec (Institut Gustave Roussy, University Paris-Saclay, Paris, France).