Cardiac angina is a major source of morbidity, affecting more than 5% of the U.S. population.2 It leads to more than 1.5 million hospitalizations and $190 billion in costs yearly.3 Obstructive coronary artery disease (CAD) is the most common cause of angina. However, no obstructive CAD is found on elective coronary angiography in more than 50% of cases.4, 5 These patients with angina but no obstructive CAD are a heterogeneous group. Some have noncardiac explanations for their angina or nonobstructive epicardial abnormalities such as coronary spasm. However, many patients with angina and no obstructive CAD have microvascular dysfunction from endothelial dysfunction or microvascular obstructive disease as the cause. These patients have microvascular angina.