A 52-year-old male with idiopathic myelofibrosis of 8 years' duration developed pericardial tamponade during recovery from acute tubular interstitial nephropathia following septicaemia. Splenectomy had been performed 7 yr previously. The tamponade was relieved by pericardiocentesis and its recurrence was prevented by a minor pericardiectomy. Pathological examination, including staining for factor VIII-positive cells, demonstrated extramedullary haematopoiesis in the pericardium. In patients with myelofibrosis and increased silhouette on X-ray film, with or without clinical heart failure, echocardiographic examination is recommended in order to identify a possible pericardial effusion.