Dietary magnesium has been shown to be important for lung function and bronchial reactivity. Interest in electrolytes in asthma has so far mainly been focused upon serum potassium, especially linked to beta 2-agonist treatment. It is known that serum levels of magnesium and potassium may not correctly reflect the intracellular status. We therefore investigated whether asthmatics treated with oral beta 2-agonists had low magnesium or potassium in skeletal muscle and serum, and whether withdrawal of the oral beta 2-agonists would improve the electrolyte levels. Magnesium and potassium levels in skeletal muscle biopsies, serum and urine were analysed in 20 asthmatics before and 2 months after withdrawal of long-term oral beta 2-agonists, and for comparison in 10 healthy subjects. Skeletal muscle magnesium in the asthmatics was lower both before (3.62 +/- 0.69 mmol.100 g-1 (mean +/- SD)) and after (3.43 +/- 0.60 mmol.100 g-1) withdrawal of oral beta 2-agonists compared with the controls (4.43 +/- 0.74 mmol.100 g-1). Skeletal muscle potassium and serum magnesium did not differ between the groups. Serum potassium was significantly lower both before (4.0 +/- 0.2 mmol.L-1) and after (3.9 +/- 0.2 mmol.L-1) the withdrawal of oral beta 2-agonists compared with the control group (4.2 +/- 0.2 mmol.L-1). The asthmatics had lower skeletal muscle magnesium and lower serum potassium than the healthy controls, both with and without oral beta 2-agonists. Whether the findings are related to asthma pathophysiology or treatment is currently being investigated.