A 67 year old woman with a permanent pacemaker was admitted with pulmonary oedema and mitral valve incompetence two months after a myocardial infarction. Echocardiograms showed good left ventricular function and a large coil of apparent thrombus in the right atrium prolapsing into the right ventricle. Intermittent loss of pacemaker sensing and capture was noticed on admission and probably caused the supraventricular tachycardia and ventricular fibrillation that occurred before an exploratory bypass operation. At operation rupture of the papillary muscle was found and the mitral valve was replaced. A large piece of thrombus was retrieved from the right pulmonary artery. The right heart contained no clot and the pacemaker wire was not displaced. It is envisaged that the strand of venous thrombus was caught in the permanent pacing wire at the tricuspid valve level resulting in an unusual case of pacemaker malfunction. The eventual poor outcome was almost certainly influenced by the arrhythmias and pulmonary embolism caused by the clot and might have been avoided by early operation.