We report on two women presenting with cough and fever, 4 and 7 months, respectively, after starting breast radiation therapy following surgery for breast carcinoma. Chest roentgenogram and computed tomographic (CT) scan demonstrated alveolar opacities, initially limited to the pulmonary area next to the irradiated breast, but later migrating within both lungs. Intra-alveolar granulation tissue was found in transbronchial lung biopsies. Corticosteroid treatment resulted in dramatic clinical improvement, together with complete clearing of the pulmonary opacities on chest imaging. However, clinical and imaging relapses occurred when corticosteroids were withdrawn too rapidly; with further improvement when they were reintroduced. The reported cases clearly differ from radiation pneumonitis. They were fairly typical of cryptogenic organizing pneumonitis, also called idiopathic bronchiolitis obliterans organizing pneumonia, with the exception of the radiation therapy, partially affecting the lung, which had been performed within the previous months. Since focal radiation therapy involving the lung may induce diffuse bilateral lymphocytic alveolitis, we hypothesize that this may "prime" the lung to further injury, leading to cryptogenic organizing pneumonitis.