A 39-yr-old woman developed progressive exertional dyspnea 13 yr after receiving mediastinal irradiation for Hodgkin's disease. Chest roentgenogram showed a hyperlucent right lung. Pulmonary blood flow was markedly reduced on the right by ventilation-perfusion scanning. Pulmonary angiography showed attenuation and diffuse atrophy of the right pulmonary artery and its branches. This case represents a late and uncommon complication of mediastinal irradiation manifesting as a unilateral hyperlucent lung.