A group of 32 patients with moderately severe, chronic asthma (mean FEV1 55% of predicted), maintained on moderately high doses of inhaled corticosteroids (mean dose 1,100 micrograms/d), participated in this double-blind, placebo-controlled crossover study. The effect on pulmonary function of adding theophylline (U, once daily Uniphyl), inhaled salbutamol (S, 200 micrograms four times per day), and their combination (C) or placebo (P) was assessed on Day 14 of each treatment phase. Patients recorded peak expiratory flow, asthma symptom severity (morning and evening), and use of rescue salbutamol inhaler in daily diaries. Mean FEV1 between 0730 and 1800 h and maximum FEV1 between 0730 and 1300 h were significantly higher on U, S, and C compared with P (p < 0.006). Morning peak flow and FEV1 (0730 h) were significantly higher on U and C compared with S and P (p < 0.01). Evening peak flow was higher on U than P (p < 0.001), and C was higher than S and P (p < 0.01). Rescue salbutamol inhaler use was significantly higher on P than on U, C, or S (p = 0.0001). Patient rating of asthma symptoms during C was significantly better than on S or P (p < 0.05). Patient rating of asthma control and study phase preference was significantly higher on combination and Uniphyl alone than on placebo, the combination also being superior to salbutamol alone. Addition of Uniphyl or a combination of Uniphyl and salbutamol significantly improves pulmonary function and asthma symptoms in patients treated with high doses of inhaled corticosteroids and as-needed beta agonists.(ABSTRACT TRUNCATED AT 250 WORDS)