In a double-blind crossover study, we compared the relative effects of inhaled beclomethasone dipropionate (BDP) 800 micrograms per day and oral theophylline on the severity of bronchial hyperresponsiveness (BHR) to histamine. Daily doses of theophylline were sufficient to keep serum levels between 55 and 110 mumol/L. The subjects were 26 patients with severe asthma whose symptoms were inadequately controlled by regular treatment with inhaled salbutamol. The severity of BHR improved within 3 wk in the group treated with BDP, whereas no change occurred in the group treated with theophylline. There were no significant changes in FEV1 in either group during the study. When BDP was changed to theophylline there was a deterioration in BHR. Aerosol steroids, rather than theophylline, are the treatment of choice when reduction in the severity of BHR is the aim of treatment in patients with severe asthma.