BACKGROUND Positive expiratory airway pressure seems to dilate narrowed or collapsed airways, but this may be accompanied by a maintained and harmful increase in resting lung volume in obstructive pulmonary disease. OBJECTIVE To evaluate the influence of inhaled terbutaline and positive expiratory pressure (PEP) on airway resistance (Raw) and functional residual capacity (FRC) in bronchial asthma. METHODS Randomized crossover design, single blind with regard to inhaled medication, open with regard to PEP (PEP can be felt). METHODS Ten patients with bronchial asthma inhaled placebo and terbutaline in doses of 0.125 mg, 0.5 mg, and 1.5 mg by cone spacer combined with a facemask giving 0, 10, or 15 cm H2O PEP on separate days. FRC and Raw were measured by body plethysmography before and after inhalations. Data were analyzed by analysis of variance with terbutaline dose and PEP as factor levels. RESULTS The effect of terbutaline: Raw decreased significantly (p < 0.0001) after 0.125 mg and 1.5 mg. The FRC did not change significantly. The effect of PEP: Raw decreased, but significantly only when the dose of 1.5 mg terbutaline was excluded from the analysis. Raw decreased with PEP 10 and 15 cm H2O, mean 0.6 (95 percent CI: -1.1, -0.2) and 0.9 (95 percent CI: -1.3, -0.4) cm H2O/L/s. The FRC did not change significantly with the PEP level. CONCLUSIONS PEP only had influence on Raw when insufficient doses of terbutaline were inhaled, whereas once an efficient dose of terbutaline was administered, significant bronchodilation was achieved with or without PEP. Positive expiratory pressure did not increase FRC.