It has been postulated that patients with congestive heart failure (CHF) benefit from physiological oral magnesium supplements. In order to study the effect of such supplements upon patients with idiopathic dilated cardiomyopathy (IDC) we studied a group of 23 patients with the salient characteristics of IDC (18 male, 5 female; mean age 51 +/- 13 years [range: 23-72]; Mg group), who received oral magnesium supplements (15 mmol/day). All patients received oral ACE inhibitors and 22 were receiving digitalis and diuretics. A control group of 27 patients, matched for age, sex, symptoms, and additional treatment (22 male, 5 female; mean age 52 +/- 13 years [range 22-71]) were not given oral magnesium supplements. All these patients also received oral ACE inhibitors, 25 were receiving digitalis, and 23 were receiving diuretics. Mean serum magnesium concentration (total serum Mg) in the Mg group was 0.83 +/- 0.06 mmol/litre [mean +/- SD, range 0.69-0.99], while in the control group it was 0.83 +/- 0.07 mmol/litre [range 0.70-0.96], P = NS. Left ventricular ejection fraction (LV-EF) improved from 32 +/- 11 per cent [range 13-51] to 42 +/- 14 per cent [range 15-66] in the Mg group and from 30 +/- 13 per cent [range 10-52] to 42 +/- 11 per cent [range: 25-64] in the control group. We found that physiological oral Mg supplements did not influence serum magnesium, LV-EF, or prognosis. Furthermore, we found that 56 per cent of all patients improved when subjected to strict guidance and therapy with ACE inhibitors, digitalis, and diuretics.