BACKGROUND Magnesium has been shown to be helpful in the treatment of acute exacerbations of asthma. Conflicting data exist concerning the effect of magnesium on bronchial hyperreactivity. METHODS We performed a randomized, double-blind, placebo-controlled study to investigate the effect of intravenous magnesium sulfate on bronchial reactivity to metacholine in 30 subjects with bronchial hyperreactivity. Two days after baseline metacholine provocation, 20 subjects received 0.3 mmol/kg/h of intravenous magnesium sulfate and 10 subjects received normal saline solution. Metacholine provocation was repeated 30 minutes after the initiation of the magnesium or placebo infusion. RESULTS The difference of the postinterventional minus the baseline provocative dose of metacholine required to decrease the forced expiratory volume in 1 second by 20% (PC20) was significantly higher in the magnesium group compared with the placebo group (0.48 +/- 0.46 mg/mL versus 0.05 +/- 0.73 mg/mL, P = .028). In the magnesium group, the PC(20) significantly increased (from 0.83 +/- 0.54 mg/mL to 1.31 +/- 0.66 mg/mL, P = .0001), whereas there was no change in the placebo group (0.86 +/- 0.52 mg/mL to 0.91 +/- 0.54 mg/mL, P = .83). CONCLUSIONS In the magnesium group, 30% of the subjects reached a normal PC(20) compared with 10% in the placebo group. We conclude that intravenous magnesium sulfate significantly improved bronchial hyperreactivity and may serve as an adjunct to standard treatment.