A 50-year-old Chinese woman, non-smoker, presented with a 6-month history of increased sweating on the right side of her face, exertional chest tightness and breathlessness. Although the patient presented with increased sweating on the right, further history and examination revealed unilateral, left-sided anhidrosis, left partial ptosis and miosis consistent with Horner's syndrome. The patient was subsequently investigated with thoracic CT to assess for an apical lung mass (Pancoast tumour). A CT chest ruled out a mediastinal tumour, however, it revealed a large 60×41 mm soft tissue mass arising from the left atrium, protruding across the mitral valve into the left ventricle, suspicious of an intracardiac tumour. The patient was referred urgently for cardiothoracic assessment at a tertiary referral centre and successful open resection was performed. Histology confirmed an atrial myxoma. The patient developed postoperative atrial fibrillation but otherwise made a full recovery.