Tumours of the left atrium are usually first discovered at aecropsy. When the diagnosis has been made during life the inding has often been a fortuitous one during an operation for mitral valve disease. Campeau and David (1960) reported 45 eft atrial myxomas, of which 4 were suspected clinically, 3were confirmed by angiography, 16 were discovered during peration for mitral stenosis, and 22 were found at necropsy. Rheumatic heart disease, subacute bacterial endocarditis, and Jisseminated lupus erythematosus are the diagnoses most commonly made in these patients before death. Four cases iescribed by Goodwsi et al. (1962) had all been suspected on linical grounds before surgery. Three of the four had first 'een thought to have an inflammatory endocarditis.We report a case of primary sarcoma of the left atrium be--use it presented features which allowed a diagnosis of a left atrial tumour to be made on clinical grounds alone. The nalignant nature of the lesion was not appreciated before opera& ion, but in retrospect the relentless rather than fluctuating Aianges in thi physical signs relating to the mitral valve with he comparatively slight constitutional upset might have made Ls suspect, before the operation, that this was a sarcoma and _at a myxoma. murmur was heard in May 1965, but no diastolic murmur was audible at that time.Acute pulmonary oedema recurred in 1965. A definite diastolic murmur was heard but was not always present. A striking feature was the normal heart size on the chest x-ray films despite the severe attacks of pulmonary oedema, She was readmitted.Investigations. Electrocardiogram showed sinus rhythm and P mitrale. Phonocardiogram: mitral diastolic and systolic murmurs and third heart sound recorded at the apex A diagnosis of left atrial myxoma was made and cardiac catheterization was performed. This showed a large filling defect above the mitral valve and extending through it into the left ventricle. It was assumed that these findings confirmed the clinical diagnosis of left atrial myxoma, Surgical removal of the tumour was undertaken. An obviously malignant tumour fungating through the left atrial wall was present On opening the left atrium an extensive polypoidal and necrotic tumour appeared to arise from just above the interatrial septum The mitral valve was extensively infiltrated throughout its circum ference, and a small area of tumour extended into the posterior cusp of the mitral valve. The tumour filled the left atrium from just below the inferior pulmonary veins and almost completely obstructed the mitral valve., As much of the tumour as possible was removed A paraffin section showed a " spindle cell " sarcoma.The patient recovered slowly from the operation, but she then deteriorated, dying six weeks later.Post-mortem Findings.-The visceral and parietal pericardium were lightlv adherent. Enlargement of the heart was present without hypertrophy. A grey-white soft granular tumour mass (see Fig.) filled the mitral orifice, extending in the way described at operation Metastatic deposits ...