Pulmonary artery hypoplasia, either congenital or acquired, is a rare abnormality seen in adults. We reported the first case of adult, isolated, left pulmonary artery hypoplasia, with exertional angina and prominent ST depression in exercise stress test, that seemed to be caused by exertional hypoxemia. Several vasodilators, including nitroglycerin, prostaglandin E1, and nifedipine, were administered individually, each with subsequent hemodynamic monitoring, pulse oximetric monitoring, and exercise test. Vasodilator administrations reduced pulmonary vascular resistance (baseline, 599; vasodilators, 306, 211, and 284 dyne · sec · m2/cm5, respectively) and attenuated ST depression (by 52, 72, and 27%, respectively) but without an immediate benefit on exercise tolerance. All vasodilators except nifedipine ameliorated exertional hypoxemia (expressed by arterial oxygen saturation during peak exercise, baseline, 69%; vasodilators, 85, 78, and 65%, respectively). Additional oxygen supply after nitroglycerin administration further benefited exertional hypoxemia (arterial oxygen saturation, 96%) and exercise ST depression (attenuated by 82%).