In case mono-bronchodilator therapy does not control symptoms of COPD adequately or if regular maintenance therapy is desired, a therapeutic intervention with a combination of bronchodilators is recommended. The risks of side-effects increases with increasing dose of any drug and, therefore, the most important rationale for combination therapy is a very favourable ratio of efficacy and safety. Knowing that anticholinergic and beta-adrenergic agents achieve their bronchodilating effects by different mechanisms, in particular the combination of these agents has proven to be beneficial in the management of COPD. Based on the established clinical benefits, tiotropium is an attractive and promising agent for the first-line long-term maintenance therapy in COPD. This also implies that a therapeutic intervention with other bronchodilators will be prescribed in daily practice. At present no studies on combination therapy with short-acting agents are available. Therefore, using a double-blind, randomised, crossover design, the bronchodilator effects of single doses of ipratropium or fenoterol were compared with placebo when added on top of steady state tiotropium. Patients were pre-treated with tiotropium to achieve this pharmacodynamic steady state. Serial lung function tests (FEV1, FVC, Raw, sGaw) were conducted following add-on of the short-acting bronchodilators or placebo.