A 65-year-old woman with dry cough and severe dyspnea was admitted to our hospital; she exhibited bilateral ground-glass opacity and mediastinal lymphadenopathy on chest computed tomography (CT). She was treated with steroid pulse therapy and indicated remarkable improvement both physically and radiographically. However, her condition deteriorated 1 week later. In addition to steroid pulse therapy, cyclophosphamide pulse therapy was introduced. After she showed significant improvement, marked aggravation was indicated on her chest CT 3 weeks later. Although her respiratory condition was worsening, we estimated that general anesthesia was narrowly feasible. Biopsy of the mediastinal lymph nodes was performed, and she was diagnosed with diffuse large B cell lymphoma. Multidrug chemotherapy with rituximab was very effective, and complete remission has been maintained for 2 years. By bronchoalveolar lavage fluid (BALF) analysis and histological findings of transbronchial lung biopsy (TBLB) specimen, we determined that the pulmonary interstitial shadow was a reactive change to the lymphoma.