Despite concern over possible adverse cardiac effects of high-dose beta-agonists there have been no controlled studies of the effects of such a therapy in patients with severe chronic airflow obstruction (CAO). We therefore studied 22 CAO patients (FEV < 1 litre) with continuous ambulatory cardiac monitoring. Patients received either nebulised salbutamol (5 mg) or saline each given 4 times daily for 24 h on 2 consecutive days, single blind, in random order. Supraventricular arrhythmias were common on both saline and salbutamol days (8 vs. 9 patients, p = NS), but none were clinically apparent. There were no episodes of ventricular tachycardia. Ventricular ectopic activity was highly variable but did not significantly differ between the two study days overall or between specific periods after nebulised salbutamol or saline. Serum potassium (mean) fell by 0.23 (SD 0.06) mmol/l in 10 patients after salbutamol. Baseline FEV1, PaO2, PCO2 were not predictive of arrhythmias or ectopic activity. While occasional adverse effects cannot be excluded, we conclude that high-dose salbutamol does not lead to any general increase in arrhythmogenic potential in severe CAO.