In a double-blind, placebo-controlled study, 273 patients with suspected acute myocardial infarction (AMI) were randomized to receive either 48-h magnesium (Mg) or placebo therapy intravenously, initiated immediately on admission to hospital. We describe the results from a 1-year survey in 270 of the patients, who were available for follow-up. Patients were equally divided: 135 received Mg and 135 received placebo. Mg treatment was associated with a marked reduction in 1-year death rate from 32% in the placebo group to 20% in the Mg group (p = 0.018). If only death from ischemic heart disease is considered, the figures were 28% in the placebo group as opposed to 15% in the Mg group (p = 0.006). This reduction was mainly due to a reduction in mortality during the initial 30 days after inclusion in the study (17% vs. 7%), after which the difference in mortality between the two groups did not reach statistical significance (18% vs. 15%, p = 0.56). The beneficial effect of Mg on mortality was partly linked to a reduced incidence of arrhythmias (27% vs. 16%), and partly to a reduced incidence of infarction (63% vs. 48%) during the initial hospitalization. However, factors unknown to us were also involved, as revealed by a remaining statistically significant partial regression coefficient, when sex, age, cardiovascular history, development of AMI, and development of arrhythmias were considered. It is concluded that intravenous Mg treatment is beneficial to patients with acute ischemic heart disease and should be adopted as part of the routine treatment of these patients.