Although the development of effective treatments for patients with chronic obstructive pulmonary disease (COPD) has not been seen as a high priority, the past decade has seen a substantial increase in the number of clinical studies examining different treatments for this disease. Large studies are needed to adequately assess the effectiveness of treatment because of the chronic nature of the disease and the intermittent occurrence of some key outcomes such as exacerbations. Data from randomised controlled trials show that treatment improves exercise performance by increasing lung volume rather than changing expiratory flow. Although assessment of lung function remains the cornerstone of drug assessment, improvements in health status, the number of exacerbations and admissions to hospital are now recognised as important treatment outcomes. Randomised controlled trial data provide the best evidence for treatment efficacy, but results of these studies can be affected by differences in inclusion criteria and patient dropout during the study. Bronchodilator reversibility testing does not reliably define subgroups that will respond to a particular treatment. Carefully done and adequately powered clinical trials continue to inform, not only our views about treatment, but also our understanding of COPD and how it is best assessed and managed. Ensuring that these expensive studies are done objectively to the highest standard is an important goal for the next decade.