Pulmonary tumour microembolism is a recognised cause of respiratory failure in patients with cancer, but is frequently under recognised. We report the case of a 42-year-old female patient admitted because of rapidly progressive severe dyspnoea with a normal chest X-ray (CXR). Initial investigation and imaging was inconclusive, with normal computerised tomography (CT) scans and pulmonary angiotomography. Videothoracoscopic biopsy was performed. Microscopic examination revealed metastatic adenocarcinoma in the lungs, with pulmonary vasculature showing numerous microemboli in small arterioles. The site of origin was consistent with a primary breast adenocarcinoma resected three years previously. Despite treatment in the Intensive Care Unit, the patient died 72 hours after admission. Active consideration and investigation for malignancy should always be considered in the differential diagnosis of patients with severe unexplained dyspnoea.