Elevation of the ST segment in the right precordial leads may be associated with electrocardiographic signs of acute cor pulmonale. This sign, which we have observed in 77 cases of moderate to severe pulmonary embolism (greater than 1.3 per cent of cases), is a very early but transient sign of usually moderate to severe pulmonary embolism. In the presence of this sign, the diagnosis of pulmonary embolism with acute cor pulmonale should be proposed at the same time as more common aetiologies such as coronary insufficiency and pericardial disease.