This randomised, double-blind, double-dummy, crossover, placebo-controlled study assessed the acute tolerability of budesonide/formoterol in a single inhaler (Symbicort Turbuhaler, AstraZeneca) administered as a high dose. Fourteen patients with asthma receiving budesonide/formoterol maintenance treatment (two inhalations of 160/4.5 microg twice daily) inhaled 10 additional doses of budesonide/formoterol 1600/45 microg (total daily dose including morning dose of maintenance treatment 1920/54 microg) or formoterol 45 microg (Oxis Turbuhaler, AstraZeneca; total daily dose including morning dose of maintenance treatment 54 microg formoterol) or placebo in addition to the morning dose of maintenance treatment on 3 separate study days. Serum potassium, pulse rate, blood pressure and ECG were assessed at regular intervals over a 12-h period following dosing. Blood glucose and plasma lactate were assessed over 3 h following dosing. Changes in serum potassium, pulse rate, blood pressure, QTc, blood glucose and plasma lactate occurring with budesonide/formoterol, though statistically significantly different from placebo (P<0.05), were considered clinically unimportant. No clinically relevant differences were identified between active treatments. In conclusion, budesonide/formoterol in a single inhaler is well tolerated at high doses such as might be used by patients using budesonide/formoterol for relief of symptoms of asthma.