Three cases of intracardiac tumour are reported in this paper and are of interest, apart from the comparative rarity of the condition, because in each case the clinical signs and symptoms suggested the presence of another lesion and the tumour was only discovered post-mortem.The first case, with a round-cell and spindle-cell sarcoma of the right auricle, showed in life the signs and symptoms of Ayerza's syndrome. The second, with a pedunculated myxomatous tumour of the left auricle, which projected through the mitral valve, was diagnosed as mitral stenosis. The third, with a tumour only in the sense that it was an intracardiac swelling, had a mycotic aneurysm arising at the attachment of the anterior cusp of the aortic valve, extending downwards into the interventricular septum and bulging into the right ventricle; the aortic valve was the seat of a bacterial endocarditis: in life the bulging into the right ventricle had obstructed the pulmonary valve, and a diagnosis of pulmonary stenosis and rheumatic carditis had been made.SARCOMA OF RIGHT AURICLE Case 1. A woman, aged 45 years, was admitted 30/7/41 complaining of increasing shortness of breath on exertion, of blueness of the lips for five months, and of a cough with scanty sputum for the past four weeks. Her doctor stated that auricular fibrillation had developed within the last week. She had always been strong and healthy except for a tendency to winter cough for the past five or six years. No history of rheumatic fever or chorea.On examination she was an obese woman with intense plum-coloured cyanosis but without dyspncea at rest. The heart appeared normal in size. Auricular fibrillation at 84 a minute was present. There were no murmurs. The blood pressure was 120/80. The lungs were emphysematous without any added sounds. The cervical veins were not overfilled, and there was no cedema of the subcutaneous tissues and no enlargement of the liver or spleen. The appearance of the patient suggested polycythemia, but her blood count showed 4-7 million red blood corpuscules and 94 per cent hemoglolin, with a colour index of 1b0. The urine was normal except for a trace of albumin. An electrocardiogram confirmed the presence of auricular fibrillation and showed low voltage of the QRS complexes and a flat T wave in lead III.There was no abnormal axis deviation.