Radiation therapy is commonly used for the treatment of lung cancer. However, radiation pneumonitis frequently occurs as a complication of the radiation therapy. Although corticosteroids are widely used for the treatment of radiation pneumonitis, they are not always effective. In this report, we used cyclosporin A in the treatment of a patient suffering from steroid-refractory radiation pneumonitis. To our knowledge, this is the first report in which cyclosporin A was successfully used in the treatment of radiation pneumonitis.