In a double-blind, placebo-controlled study, 47 patients with ischemic heart disease and acute myocardial infarction were allocated to 3 months' treatment with peroral magnesium (15 mmol/d) or placebo. Before, during, and after treatment, blood samples were taken to determine serum concentrations of cholesterol; triglyceride; high-density, low-density, and very-low-density lipoprotein; apolipoprotein A1 and B; and magnesium. We found a 13% increase in molar ratio of apolipoprotein A1:apolipoprotein B after magnesium treatment, as compared with a 2% increase in the placebo group (for mean differences between changes of the magnesium and the placebo groups). This increase was caused by a decrease in apolipoprotein B concentrations, which were reduced by 15% from 1.44 to 1.23 mmol/L in the magnesium group as compared with a slight increase in the placebo group. Triglyceride, and thereby very-low-density lipoprotein concentrations decreased by 27% after magnesium treatment (from 2.41 to 1.76 mmol/L, and from 1.1 to 0.79 mmol/L, respectively) as compared with much smaller decrements in the placebo group. Likewise, we found tendencies toward an increase in high-density lipoprotein cholesterol and in high-density lipoprotein cholesterol ratio/(low-density lipoprotein cholesterol:very-low-density lipoprotein cholesterol) after magnesium treatment. The observed findings support the hypothesis that magnesium deficiency might be involved in the pathogenesis of ischemic heart disease by altering the blood lipid composition in a way that disposes to atherosclerosis.