A 54-year-old woman presented to the emergency department with increasing shortness of breath over four days. She reported an increased cough over the previous two months and that her daughter recently had an upper respiratory tract infection. The patient had metastatic breast cancer (estrogen-receptor positive, human epidermal growth factor receptor 2/neu negative). She had received first-line paclitaxel chemotherapy four years earlier, but severe neuropathy developed; her regimen was changed to doxorubicin weekly, with good response and a maximum lifetime cumulative dose of 450 mg/m2. Restaging performed three months before this visit showed progression in the liver, resulting in a new regimen that included everolimus and exemestane. In light of her ongoing shortness of breath, the patient had a bronchoscopy to rule out lymphangitic spread and an infectious cause; the bronchoscopy findings were negative for either cause. She was admitted to the internal medicine service for further evaluation of the unexplained dyspnea and high BNP levels. She improved substantially after diuretic therapy.