A 49 year old man presented with severe cyanosis and dyspnoea on exercise. Clinical examination together with echocardiography, cardiac catheterisation, and angiography showed a balloting tumour in the right atrium, intermittently occluding the tricuspid ostium, and an atrial right to left shunt. At operation a pedunculated vascular tumour was found with a broad base which was embedded in the atrial wall and continued into the interventricular septum. Histological examination showed angiosarcomatous features and signs of a less than radical excision. The patient, who made an uneventful recovery, was given postoperative radiotherapy. After 36 months there are no signs of recurrence or metastasis.