The efficacy and side-effects of a new corticosteroid aerosol, budesonide and beclomethasone dipropionate were assessed in 30 patients with chronic asthma in a double-blind cross-over study. Budesonide was administered, 200 micrograms twice daily, from a conventional pressurized aerosol with a tube spacer extension attached and beclomethasone was given via a conventional inhaler in the recommended four times daily dose of 100 micrograms, each treatment being administered for one month. No clinically significant differences were found between the two treatments and no significant side-effects were observed.