The Unilateral Absence of a Pulmonary Artery(UAPA) is an uncommon congenital anomaly. The diagnosis may be suspected from chest roentgenogram and confirmed by pulmonary arteriography. UAPA is frequently associated with other anomalies of the cardiovascular system such as patent ductus arteriosus and tetralogy of Fallot. When it occurs as an isolated anomaly, patients are more frequently asymtomatic, while and only a minority of cases develop pulmonary hypertension. We report an asymptomatic case of UAPA in an 1-year-old girl. Her chest X-ray revealed reduction of volume of right lung with shifting of the heart and mediastinum to the right side and elevation of the right hemidiaphragm. There was the differrence in the vascularity between the two lung fields, and the right hilar shadow was absent. Her lung perfusion scan revealed total perfusion defect of right lung. The diagnosis of UAPA was confirmed by pulmonary arteriography