In an epidemiological and follow-up survey on 712 patients, 52 subjects with proven ischaemic heart disease were matched with and compared to 52 coronary-prone subjects with similar major cardiovascular risk factors (high-risk controls, HR) as well as to 52 patients at low cardiovascular risk (low-risk controls, LR). HR and LR patients were all free of overt ischaemic heart disease. Both the average daily dietary magnesium intake and the 24 h renal magnesium output were slightly higher in HR as compared to LR and ischaemic heart disease patients. No difference could be observed between the three groups with respect to serum magnesium levels, whereas erythrocyte magnesium levels were lower in ischaemic heart disease patients than in LR (P = 0.089) and to HR (P = 0.042). Ischaemic heart disease patients below 60 years had significantly lower erythrocyte magnesium levels than older (> 60 years) ischaemic heart disease patients. Lower erythrocyte magnesium levels in ischaemic heart disease patients were also associated with an increased incidence of cardiac events in the follow-up period and with a more unfavourable outcome. A pilot phase 6 month open trial of oral magnesium supplementation in nine ischaemic heart disease patients with low erythrocyte magnesium levels led to significant increases of erythrocyte magnesium in these patients, and to an impressive decrease of anginal attacks and nitrate consumption, as well as to a lesser degree of ST segment depression on surface ECG obtained at exercise testing in seven patients.