A 43-year-old woman with breast cancer who was on neoadjuvant chemotherapy presented with cough, sputum and mild fever. High-resolution computed tomography showed diffuse ground glass opacities in bilateral lungs and subpleural patchy consolidations. Initially, she was thought to have pneumonia or interstitial lung diseases such as drug-induced pneumonitis and treated with antibiotics and steroids. She subsequently got breast cancer surgery because of disease progression, and concurrent thoracoscopic lung biopsy revealed metastatic carcinoma of the lung from breast cancer. The diagnosis of suspected interstitial lung disease can be made without lung biopsy, but malignancy should always be considered and lung biopsy should be performed in the absence of a definitive clinical diagnosis.