The purpose of this study was to investigate the effects of 500 μg roflumilast, taken once daily for 12 weeks, on airway physiology during rest and exercise in patients with moderate-to-severe chronic obstructive pulmonary disease. This randomised, double-blind, placebo-controlled, parallel-group study was conducted in 250 patients with a post-bronchodilator forced expiratory volume in 1 s (FEV(1)) of 30-80% predicted and a functional residual capacity of ≥ 120% pred. Pre- and post-bronchodilator spirometry and body plethysmography, and pre-bronchodilator constant work rate cycle exercise at 75% of peak work rate were evaluated. Exercise measurements included ventilation, breathing pattern, inspiratory capacity (IC) and arterial oxygen saturation measured by pulse oximetry (S(p,O(2))). Compared with placebo, 12 weeks of treatment with roflumilast was associated with: small but progressive increases in pre- and post-bronchodilator FEV(1) and FEV(1)/forced vital capacity; small decreases in specific airway resistance; and no significant changes in resting vital capacity, IC or measurements of lung hyperinflation. There was no treatment effect on exercise endurance time. At a standardised exercise time after roflumilast, compared with placebo, IC increased by 0.12 L (p = 0.008) and S(p,O(2)) increased by 0.7% (p = 0.020); peak ventilation increased by 1.9 L · min(-1) (p = 0.014). Roflumilast treatment was associated with progressive improvement of airway function but not lung hyperinflation. Newly described non-bronchodilator effects of roflumilast included small but consistent improvements in air trapping and S(p,O(2)) during exercise.