In a pilot study at 14 patients with coronary heart disease (CHD) and left-ventricular dysfunction (left ventricular enddiastolic volume [LVEDV] > or = 100 ml), who actively participated in an ambulatory cardiac sports group, left ventricular endsystolic volume (LVESV), LVEDV and duration of exercise were analyzed by echocardiographic and ergometric tests. An initial workup was followed by a 4 week double blind treatment phase, in which magnesium orotate 3 x 1 g or placebo was given additionally to medication taken prior to the study. At the end of this phase a concluding workup was performed. Magnesium orotate decreased significantly (p = 0.016) LVESV, increased significantly (p = 0.035) EF, decreased in tendency (p = 0.054) LVEDV and increased significantly (p = 0.011) exercise duration. The study gives references to favourable effects of oral magnesium orotate to left ventricular function and exercise tolerance in patients with CHD.