To elucidate the contribution of magnesium to bronchial hyperreactivity in patients with stable bronchial asthma, magnesium concentrations in serum (S-Mg), erythrocytes (R-Mg), and lymphocytes (L-Mg) were measured in 25 patients with bronchial asthma (BA group) and 9 age-matched healthy subjects (control group). A parenteral magnesium loading test, a continuous low-dose magnesium infusion of 0.2 mEq/kg over 4 hr, was performed in 10 of 25 asthmatic patients and in the control group. R-Mg was significantly lower in the BA group than in the control group (4.96 +/- 0.47, 6.13 +/- 0.62 mEq/L, p < 0.001, respectively), although S-Mg (2.4 +/- 0.1, 2.4 +/- 0.2 mg/dL) and L-Mg (1.28 +/- 0.26, 1.15 +/- 0.13 microg/mg/protein) were not significantly different between the two groups. Magnesium deficiency in total body stores was revealed in 40% of patients (4/10 patients) and 11% of control subjects (1/9 subjects) by parenteral magnesium loading test. The ratio of magnesium retention to urinary excretion through the parenteral magnesium loading test showed a significant inverse correlation with R-Mg (r = -0.78, p < 0.01). Bronchial reactivity to inhaled methacholine had a significant inverse correlation with R-Mg (r = -0.42, p < 0.05). We conclude that 40% of asthmatic patients demonstrated magnesium deficiency, and that the low magnesium concentration in erythrocytes reflects decreased magnesium stores in patients with bronchial asthma.