Endothelial dysfunction is an important pathophysiologic mechanism in the progression of heart failure. The objective of the present study was to determine the effects of acute and chronic oral magnesium supplementation on endothelial function in patients with symptomatic heart failure. Twenty-two symptomatic chronic heart failure patients were randomized to receive 800 mg oral magnesium oxide daily or placebo for 3 months. Data collected included large and small arterial elasticity/compliance, hemodynamic parameters, exercise capacity, and quality-of-life score at baseline, 1 week, and 3 months. Patients who received magnesium had improved small arterial compliance at 3 months from baseline compared with placebo. This study suggests that chronic supplementation with oral magnesium is well tolerated and could improve endothelial function in symptomatic heart failure patients.