We report the case of a 58-year-old woman who was considered to have died of cyclophosphamide (CPA)-induced late-onset lung disease. She underwent a right-sided mastectomy due to breast cancer at the age of 50 followed by daily administration of 50 mg CPA and 20 mg tamoxifen for 2 years. A refractory cough and dyspnea began at the age of 56. Chest radiographs revealed diffuse infiltrates and pleural thickening. Her vital capacity was markedly reduced. Corticosteroid therapy was ineffective. The disease rapidly progressed with occasional episodes of pneumothoraces to her death. Postmortem examination revealed pulmonary fibrosis with marked elastosis.