A 51-year-old woman presented with a 3-week history of cough, sputum, and low-grade fever. Atypical pneumonia was suspected, based on infiltrative shadows on chest X-rays and the results of blood tests, but symptoms persisted despite antimicrobial treatment. We suspected metastatic lung cancer because ground-glass opacity around nodular shadows and a reversed halo sign were detected on computed tomography of the chest. Testing for tumor markers showed high levels of serum CA15-3 and BCA225. We thought she had breast cancer, but the pathological diagnosis from transbronchial lung biopsy was organizing pneumonia. Immunohistochemical staining for CA15-3 showed a positive reaction in alveolar epithelial cells. The patient was effectively treated with corticosteroid and immunosuppressant, and tumor marker levels decreased in accordance with improvements to such clinical observations as symptoms and radiological findings. We attributed the high level of serum CA15-3 to organizing pneumonia. This is the first report of a patient with cryptogenic organizing pneumonia showing increased levels of tumor markers for breast cancer.