The haemodynamic effect of a long-acting vasodilator isosorbide dinitrate has been studied in 10 patients after an acute myocardial infarct, all of whom had evidence of left ventricular failure. Left ventricular filling pressure measured as the mean pulmonary artery wedge pressure was raised in all patients and fell significantly from 20+/-6 to 13+/-5 mmHg (P less than 0-001) within 10 minutes of sublingual isosorbide dinitrate. This 35 per cent fall in left ventricular preload was accompanied by significant fall in mean pulmonary artery pressure from 30+/-7 to 20+/-4 mmHg (P +less than 0-001) and mean right atrial pressure from 11+/-3 to 6+/-2 mmHg but cardiac output measured by thermodilution was unchanged. Mean systemic blood pressure was also significantly reduced. This improvement in left ventricular performance resulting from a reduction in left ventricular filling pressure and systemic blood pressure indicates that there may be a place for long-acting vasodilator in the treatment of acute myocardial infarction.