The influence of positive expiratory pressure (PEP) applied during inhalation of a beta 2-agonist in treatment of bronchial asthma was investigated in a randomized crossover study with two-week treatment periods. In one period, two puffs (0.5 mg) of terbutaline was given from a metered dose inhaler and inhaled through a device consisting of a conespacer connected to a facemask giving PEP (10 to 15 cm H2O). In a second period, terbutaline 0.5 mg was inhaled similarly but without PEP, and in a third period placebo spray was inhaled with PEP. Treatments were given three times daily. Peak expiratory flow (PEF) was measured before and after each inhalation and symptom scores for dyspnea, cough, and mucus production were noted in a diary. All treatments increased PEF significantly (p less than 0.0001). The mean increase was 32 L/min during treatment with terbutaline and PEP. This was greater than the increase of 25 L/min during terbutaline treatment (p = 0.005). The increase in PEF during terbutaline treatment was significantly higher than the achieved 18 L/min during PEP (p = 0.026). The study showed improved bronchodilation when PEP was combined with inhalation of beta 2-agonist compared with beta 2-agonist alone.