Background Experimental evidence indicates that magnesium sulfate may have potential cardioprotective properties as an adjunct to coronary reperfusion. The present study was designed to examine the hypothesis that magnesium might have beneficial effects on left ventricular (LV) function and coronary microvascular function in patients with acute myocardial infarction (AMI). Methods and Results The study population of 180 consecutive patients with a first AMI (anterior or inferior) underwent successful primary coronary intervention. Patients were randomized to treatment with either intravenous magnesium (magnesium group, n=89) or normal saline (control group, n=91). Pre-discharge left ventriculograms were used to assess LV ejection fraction (LVEF), regional wall motion (RWM) within the infarct-zone and LV end-diastolic volume index. The Doppler guidewire was used to assess coronary flow velocity reserve (CFVR) as an index of coronary microvascular function. Magnesium group subjects showed significantly better LV systolic function (LVEF 63±9% vs 55±13%, p<0.001; RWM: -1.01±1.29 SD/chord vs -1.65±1.11 SD/chord, p=0.004), significantly smaller LV end-diastolic volume index (63±17 ml/m² vs 76±20 ml/m², p<0.001), and significantly higher CFVR (2.95±0.76 vs 2.50±0.99, p=0.023) than controls. Conclusion Magnesium sulfate as an adjunct to primary coronary intervention shows favorable functional outcomes in patients with AMI. (Circ J 2004; 68: 23 - 28)