An 80-year-old female presented with dyspnoea for two weeks. She had a history of atrial fibrillation, breast cancer in remission diagnosed and treated in 1990s, chronic obstructive pulmonary disease (COPD), mechanical mitral valve on chronic warfarin therapy with good compliance, and newly diagnosed lung cancer three weeks before presentation. She had a remote 35 pack-year smoking history. On arrival, vital signs were stable except for tachycardia with a heart rate of 110 beats/min. Oxygen saturation was >90% on room air. Physical exam was notable for bilateral wheezing in the setting of COPD. Complete blood count and basic metabolic panel were unremarkable. International Normalized Ratio (INR) on admission was supratherapeutic at 4.9 (goal for the patient was 3.0–3.5).Computerized tomography angiogram was performed which ruled out pulmonary embolism but revealed an incidental mass in left atrium extending into left atrial appendage (Figure ). Transoesophageal echocardiogram showed a mass (6.1 cm × 2.6 cm) filling left atrial appendage extending into left atrium impinging on mitral valve (Figure ). The mass was thought to be either a thrombus or tumour with high suspicion of the former., Cardiac surgeons recommended against surgery due to multiple co-morbid conditions. Anticoagulation was provided initially with heparin infusion switched to enoxaparin two days later. Dyspnoea improved significantly over two weeks. Follow-up echocardiogram four weeks later showed a reduction in mass to about half the original size which favoured the diagnosis of thrombus over tumour. The oncology proceeded with outpatient chemotherapy for lung cancer and enoxaparin was continued. The development of left atrial thrombus while being on warfarin was likely due to new-onset lung cancer. Low molecular weight heparin may present a favourable anticoagulation strategy in this patient cohort.Computerized tomography angiogram scan showing left atrial mass (4.2 cm × 2.6 cm) shown with measurements.Two-dimensional transoesophageal echocardiogram showing large (6.1 cm × 2.6 cm) left atrial mass (red arrow) extending into left atrial appendage.Consent: The author/s confirm that written consent for submission and publication of this case report including image(s) and associated text has been obtained from the patient in line with COPE guidance.Conflict of interest: none declared.