To study whether magnesium (Mg) deficiency is present in patients with variant angina, 24-hour Mg retention of low dose Mg (0.2 mEq/kg lean body weight) administered intravenously over 4 hours in 20 patients with variant angina was examined. No patient had received calcium antagonists before or during the study. All had attacks of chest pain associated with ST elevation on electrocardiograms. Twenty-one subjects without ischemic heart disease were studied as control subjects. Ten patients with variant angina were restudied 10 to 529 days (mean 235 +/- 30) after the treatment with calcium antagonists (diltiazem 120 to 240 or nifedipine 40 to 80 mg/day), which resulted in complete suppression of anginal attacks. The mean serum Mg concentrations in the patients with variant angina and the control subjects were 2.1 +/- 0.05 and 2.1 +/- 0.03 mg/dl, respectively (difference not significant). However, 24-hour Mg retention in the patients with variant angina was 60 +/- 5%, while that in the control subjects was 36 +/- 3% (p less than 0.001), suggesting that Mg deficiency is present in at least some patients with variant angina. The mean serum Mg concentrations before and after calcium antagonist treatment in 10 patients with variant angina were 2.1 +/- 0.09 and 2.1 +/- 0.07 mg/dl, respectively (difference not significant). However, 24-hour Mg retention decreased significantly (p less than 0.01) from 60 +/- 6 to 34 +/- 7% after the treatment. There is Mg deficiency in many patients with variant angina and it is corrected after treatment with calcium antagonists.