In patients with HHT, AVMs are most commonly seen in pulmonary circulation. Conversely, HHT accounts for up to ≥ 70% cases of pulmonary AVMs. Dyspnea on exertion and hemoptysis are the most common symptoms associated with pulmonary AVMs. Intrapulmonary right to left shunting of blood may cause substantial hypoxemia and may manifest as clubbing and/or polycythemia. In the presence of basal pulmonary AVMs, platypnea may occur. In patients with platypnea and in some even without platypnea, orthodeoxia can be demonstrated, which is defined as a drop in oxyhemoglobin saturation by ≥ 2% when assuming an erect posture from the supine position. It is a characteristic feature of basal pulmonary AVMs; other differential diagnoses include hepatopulmonary syndrome and atrial septal defects. A bruit or murmur over the pulmonary AVMs may be heard in one-third of patients. Pulmonary AVMs usually remain stable in their size, but nearly one-fourth of these AVMs may enlarge slowly at the rate of 0.3 to 2 mm/y. The walls of these AVMs are thin, which may result in their rupture into the bronchus, causing hemoptysis, or into the pleural cavity, resulting in hemothorax. Pulmonary hypertension may be encountered in nearly 10% of patients with HHT, which may be pathophysiologically similar to heritable (idiopathic) pulmonary arterial hypertension. It may also arise in the presence of hepatic AVMs with resultant portopulmonary hypertension or group 2 pulmonary hypertension (because of high output left heart failure). In some patients, pulmonary hypertension may increase in severity from baseline or appear newly after resection or embolization of pulmonary AVMs. Redistribution of the pulmonary blood flow after resection or embolization is proposed as the mechanism in the latter group of patients.