Background: Inhaled corticosteroids and oral theophylline are effective treatments for moderate asthma. Objective: We sought to compare the benefits and adverse reactions of theophylline and aerosol beclomethasone spray. Methods: A multicenter, double-blind, double-placebo, randomized, controlled trial of 1-year duration was performed. Seven hundred forty-seven patients with asthma received either beclomethasone dipropionate aerosol spray (84 μg four times per day) or sustained-release theophylline twice per day in doses adjusted for optimum control of the disease. The main outcome measures were daily diary of symptoms and peak flow rates (recorded on a mark-sense computer-readable form); supplemental bronchodilator use; doctor's office or hospital visits and absence from work or school; spirometry; methacholine testing; adverse experiences; and cortisol blood measurements. Results: Both treatment strategies reduced symptoms promptly and achieved low absenteeism from work or school and low rates of emergency treatment for asthma. Both maintained nearly normal pulmonary function. Beclomethasone was statistically significantly more effective in reducing symptoms, supplemental bronchodilator and systemic glucocorticoid doses, bronchial hyperresponsiveness, and eosinophilia. However, the magnitude of these differences was small. Theophylline caused more headache, nervousness, insomnia, and gastrointestinal distress, and more patients discontinued treatment because of side effects. Beclomethasone caused more oropharyngeal candidiases and hoarseness and reduced morning plasma cortisol levels before and after cosyntropin. It reduced the rate of growth in children. No new cataracts or glaucoma developed. Conclusion: Theophylline effectively controlled symptoms at lower than the customarily recommended blood level. The risk/benefit profiles of these agents suggest that inhaled corticosteroids may be the preferred agent for most adult patients and for some children. (J Allergy Clin Immunol 1998;101:14-23.)