In 76 patients with angiographically proven coronary heart disease and in 34 without angiographically demonstrable changes in the coronary vessels, magnesium concentration in serum was measured. The results were compared with the coronary angiographic findings, the left ventricular pump function and the known risk factors. The patients with coronary disease had on average lower magnesium values in serum than the control group (0.835 +/- 0.075 compared with 0.89 +/- 0.08 mmol/l, P less than 0.002). Separation of the coronary patients according to ventricular function showed that only patients with reduced left ventricular function (ejection fraction less than or equal to 55%) had low serum magnesium concentrations when compared with the control group (0.79 +/- 0.075 compared with 0.89 +/- 0.08 mmol/l, P less than 0.001). Patients with coronary disease and normal left ventricular function (ejection fraction greater than 55%) did not differ significantly in their magnesium concentration from the normal controls. The results show that in patients with coronary heart disease lowered serum magnesium concentrations are generally only found when left ventricular function is reduced. The high incidence of sudden cardiac death could be connected with the increased occurrence of low magnesium values in serum of patients with coronary heart disease and diminished ventricular function.