We present a patient with acute retrosternal pain and dyspnoea. Clinical examination revealed a woman in shock with a loud holosystolic murmur and congested jugular veins. Echocardiography showed severe mitral insufficiency. At transoesophageal echocardiography a large mass was seen in the right atrium. The differential diagnosis was a thrombus or an intracardiac tumour. Subsequent histology revealed a non-Hodgkin lymphoma in the right atrium, the wall of the left atrium extending to the mitral valve and further location in the pelvis and duodenum. The discussion incorporates a brief overview of the literature.