B-type natriuretic peptide (BNP) has favourable effects on left ventricular remodelling, including antifibrotic and antiapoptotic properties. We tested the hypothesis that infusion of BNP after an acute myocardial infarction would reduce left ventricular systolic and diastolic volumes and improve left ventricular ejection fraction compared with placebo.A total of 58 patients who underwent successful revascularisation for an acute ST elevation anterior myocardial infarction were randomised to receive 72-hour infusion of BNP at 0.006 µg/kg/min or placebo. Left ventricular end diastolic and systolic volumes and left ventricular ejection fraction were measured at baseline and at 30 days by multigated acquisition scan. Left ventricular infarction size was measured by cardiac MRI.BNP infusion led to significantly higher BNP levels and plasma cyclic guanosine monophosphate at 72 hours. No significant difference in change of left ventricular volumes or ejection fraction from baseline to 30 days was observed between groups. Although left ventricular infarction size measured by cardiac MRI was not significantly different between BNP infusion versus placebo (p=0.39), there was a trend towards reduced infarction size in patients with a baseline ejection fraction of <40% (p=0.14).Infusion of BNP in patients with an anterior myocardial infarction did not affect parameters of left ventricular remodelling. Patients treated with BNP who had a baseline left ventricular ejection fraction of <40% had a trend towards reduced left ventricular infarction size compared with placebo. These results do not support the use of intravenous BNP in patients after recent myocardial infarction.NCT00573144.