Introduction: The role of magnesium in treating arrhythmias due to acute myocardial infarction (AMI) has been controversial. Most of the studies have reported a significant reduction in the mortality and frequency of arrhythmias in patients with AMI, who received magnesium therapy, in comparison with control with AMI, who did not receive magnesium treatment. We, therefore, decided that we would evaluate the effect of intravenous magnesium supplement therapy in patients admitted for acute myocardial infarction and if this would be helpful in reducing the morbidity and mortality in patients. Materials and Methods: The study was carried out as a prospective randomized controlled trial of magnesium therapy in acute myocardial infarction. Hundred patients with acute myocardial infarction were randomly divided into two groups of 50 patients each, one for trial and other for placebo therapy. Control group consisted of 20 healthy volunteers of the same age group. All of the 100 patients included in the study had reported within 12 hours of the onset of chest pain. Blood samples for estimation of serum magnesium were taken on admission and on days 2 and 5. Results: The mean age in the Group I and II were 59.52 years ± 15.03 and 59.14 years ± 13.41, respectively. Serum magnesium level on admission was higher in the healthy controls than in the patients of acute myocardial infarction (P < 0.0001). Fourteen percent patients of Group I had conduction disturbance, whereas 4% patients of Group II had conduction disturbances. Fifty percent patients of Group I developed heart failure, whereas only 30% patients of Group II developed this complication (P < 0.005). Incidence of cardiac arrhythmias, heart failure, and hospital mortality were significantly lower in magnesium treated group. Mortality as well as morbidity was very less in the magnesium treated group of patients as compared to placebo treated patients. Conclusions: Intravenous magnesium is a safe and effective method of reducing the frequency of arrhythmias and mortality following the acute myocardial infarction. Magnesium therapy reduces the incidence of arrhythmias and mortality even in the absence of demonstrable magnesium deficiency.