SUMMARY A 74 year old woman with mixed mitral valve disease presented with episodes of loss of consciousness and fainting. The attacks were noted to be closely related to posture, especially crouching, and this, with the clinical finding that the radial pulse disappeared before she lost consciousness, suggested the diagnosis of a ball valve thrombus. Cardiac catheterisation confirmed the diagnosis but was followed by a fall in blood pressure necessitating emergency surgery. The only position in which the circulation could be maintained was the right lateral, with steep head down tilt, and left femoro-femoral cardiopulmonary bypass had to be established in this position. Operation confirmed the presence of a ball valve thrombus and the patient recovered uneventfully.Left atrial ball valve thrombi and left atrial myxomata are well recognised causes of circulatory obstruction. The former have seldom been reported: they are easily removed surgically but the diagnosis is rarely made before death. Case reportThe patient, a 74 year old housewife, had rheumatic fever as a child. In 1969 she was admitted to hospital with a right lobar pneumonia and the signs of mild mixed mitral valve disease were noted; she was in sinus rhythm and the chest radiograph showed moderate cardiac enlargement. In 1976 she suffered a right hemiplegia but recovered completely within a few days. In November 1980 she presented with a six month history of progressive congestive failure despite digoxin and diuretics, and a six week history of dizziness associated with chest pain and blackouts.On examination she was alert with peripheral cyanosis, a small amplitude regular arterial pulse, blood pressure 110/70 mmHg, a venous pressure of +8 cm, 4 cm hepatomegaly, and ankle oedema: she was apyrexial. There was a parasternal heave, an apical pansystolic murmur, a late opening snap, and a full length diastolic murmur. A chest radiograph showed cardiomegaly with left atrial enlargement, bilateral pleural effusions, and a prominent main pulmonary artery. Her electrocardiogram showed sinus rhythm with multiple atrial ectopic beats and digoxin effect. Full blood count was normal and erythrocyte sedmentation rate 10.Her episodes of loss of consciousness continued on the ward, up to 10 times daily, each beginning with the patient feeling dizzy and then rapidly losing consciousness. After 20 to 30 seconds she convulsed, and then recovered in a further 2 to 3 minutes. The episodes occurred in any position but became an invariable accompaniment to attempts to defaecate. Her congestive failure was increasingly resistant to treatment. It was thought that she was suffering from epilepsy caused by her previous cerebral infarct but her condition did not improve with anticonvulsants.Clinical examination during an attack, with simultaneous cardiographic monitoring, showed that the first event was a gradual weakening of the radial pulse with no alteration of rate or rhythm at a time when the cardiac impulse was still forcible. The patient then became unconscious, pulsel...