The study was a randomised, double-blind trial in 6213 patients with acute ST-elevation myocardial infarction (STEMI) who were assigned a 2 g intravenous bolus of magnesium sulphate administered over 15 minutes, followed by a 17 g infusion of magnesium sulphate over 24 hours (n=3113), or matching placebo (n=3100). The primary endpoint was 30-day all-cause mortality. At randomisation, patients were stratified by their eligibility for reperfusion therapy. The first stratum included patients who were aged 65 years or older and eligible for reperfusion therapy, and the second stratum included patients of any age who were not eligible for reperfusion therapy. Analysis was by intention-to-treat. At 30 days, 475 (15.3%) patients in the magnesium group and 472 (15.2%) in the placebo group had died.. No benefit or harm of magnesium was observed in eight prespecified subgroup analyses of patients and in 15 additional exploratory subgroup analyses. After adjustment for factors shown to effect mortality risk in a multivariate regression model, no benefit of magnesium was observed.