A 70 year old woman was admitted for right ventricular failure and cyanosis of recent onset. Echocardiography showed a very large, homogenous, immobile, smooth-contoured mass filling the right atrium. Right atrial pressures were raised but the other intracardiac pressures were normal at catheterisation. Right heart angiography confirmed the voluminous right atrial mass and dilatation of the hepatic veins and showed early opacification of the left heart chambers. A right-to-left shunt was confirmed by oximetry which showed significant desaturation of the blood in the left atrium and ventricle (saturation 78% in the left ventricle). The tumour was also documented by a thoracic CT scan. At surgery, a very large, malignant right atrial tumour was resected which histological examination showed to be an angiosarcoma. The interatrial septum seemed to be intact: there was no true atrial septal defect but a persistent foramen ovale was found. After resection of the tumour the right atrium was reconstructed. The initial postoperative period was uncomplicated but the patient died nine months later of metastatic disease particularly affecting the liver and brain. The clinical presentation of malignant cardiac tumour is very variable but a right-to-left interatrial shunt through a patent foramen ovale has only been reported previously in 2 cases of primary malignant tumours (a rhabdomyosarcoma and an angiosarcoma) and in 1 case of a secondary cardiac metastasis.