The aims of this study were to establish whether a magnesium (Mg) deficit indicated by a decreased urinary excretion exists and to determine whether 12-week oral Mg supplementation affects the Mg status and bronchodilator use in children with stable bronchial asthma. The effects of long-lasting Mg supplementation were investigated in 89 children 4 to 16 years of age with mild or moderate persistent bronchial asthma in a randomized, double-blind, placebo-controlled, prospective study. Each subject received one capsule of Mg citrate per day (= 7 years: 200 mg, > 7 years: 290 mg) or one capsule of placebo containing 260 mg glucose during 12 weeks. Evaluation was performed at 4-week intervals. Venous blood serum total and free Mg and urine Mg levels were determined at the beginning and end of the 12-week period. Parents recorded the number of bronchodilator doses twice daily. A urinary Mg loss (6.81 +/- 3.9 versus 2.79 +/- 1.39 mmol/day, p = 0.01) was observed in the placebo-treated persistent moderate asthmatics. Bronchodilator use was significantly higher after 8 and 12 weeks in the placebo-treated than in the Mg-treated patients with moderate asthma (31.1 +/- 1.8 versus 29.5 +/- 1.2 puffs per patient/4 weeks, p < 0.05, and 31.0 +/- 2.3 versus 29.3 +/- 0.9 puffs per patient/4 weeks, p < 0.05, respectively). Long-lasting Mg supplementation is clearly of benefit in mildly to moderately asthmatic children and is recommended as a concomitant drug in stable asthma.