A 40-year-old woman was admitted to our hospital for investigation of an abnormal chest roentgenogram. She had been well with normal exercise tolerance and had no history of pneumonia. The chest roentgenogram showed decreased vascularity of the right lung, ipsilateral small hemithorax, and shift of the mediastinum toward the right. In the right lung field, hazy and confluent micronodular shadows were seen, and the left lung was emphysematous and hyperlucent. Herniation of the left lung into the contralateral thorax was seen. Although lung perfusion scintigrams showed no detectable blood flow to the right lung, a krypton ventilation study revealed diminished but homogeneous ventilation of the ipsilateral lung. A bronchogram showed normal structure of the bronchi. Cardiac catheterization revealed normal pulmonary arterial pressure. A pulmonary angiogram showed no right pulmonary artery, and absence of other cardiovascular anomalies. Therefore, the diagnosis of isolated absence of the right pulmonary artery was confirmed. An aortic angiogram demonstrated collateral vessels originating from one of the bronchial arteries, the infradiaphragmatic artery, and the right coronary artery. Chest CT disclosed cystic changes in the right lung field.