['To evaluate magnesium deficiency during and after acute myocardial infarction (AMI) and the role of intravenous magnesium therapy given in the early postinfarction period.', 'One hundred patients with AMI were randomly assigned to 2 equal groups and monitored over a 4-week period. The placebo group received intravenous dextrose solution and the trial group was given 15 g intravenous magnesium (62 mmol) over a 48-hour period. Serum magnesium levels were measured on days 1, 2, 4, and 6 after admission by calorimetry with methyl thymol blue.', 'The groups were comparable in prevalence of risk factors for coronary artery disease and other acute parameters of AMI. The serum magnesium levels of a group of 50 controls were higher (1.61 +/- 0. 21 vs 1.23 +/- 0.27mEq/L) than in patients with AMI (P <.001). There was an increase in serum magnesium levels in the trial group on day 2 (1.73 +/- 0.27 vs 1.29 +/- 0.28 mEq/L; P <.001) as well as on day 4 (1.62 +/- 0.25 vs 1.38 +/- 0.36 mEq/L; P <.001). The trial group also showed significantly lower incidence of arrhythmias (8% vs 34%) and death (4% vs 20%). The mortality rate from pump failure was reduced in the trial group (4% vs 14%).'] Is benefits of magnesium in acute myocardial infarction : timing crucial?