Previous studies have demonstrated that magnesium supplementation improves endothelial function in patients with coronary artery disease (CAD). However, the impact on clinical outcomes, such as exercise-induced chest pain, exercise tolerance, and quality of life, has not been established. In a multicentre, multinational, prospective, randomised, double-blind and placebo-controlled trial, 187 patients with CAD [151 men, 36 women; age 63 ± 10 years (mean ± SD), range 42–83] were randomised to receive either oral magnesium 15 mmol twice daily (total magnesium 365 mg provided as magnesium citrate) or placebo for 6 months. Magnesium treatment significantly increased exercise duration time compared with placebo (8.7 ± 2.1 vs. 7.8 ± 2.9 min), and lessened exercise-induced chest pain (8% vs. 21%). Quality-of-life parameters significantly improved in the magnesium supplementation group.