Reversibility of airflow obstruction was considered for many years to be a hallmark of asthma. It is now clear that many patients with chronic airflow obstruction, often labelled as chronic obstructive airways disease or chronic bronchitis and emphysema, show some bronchodilation following various drugs. At present there is no clinical or physiological factor which reliably predicts the bronchodilator response of such patients to a given medication, and individual testing of these drugs is necessary. However, the literature on bronchodilator responses in chronic airflow obstruction is confused. There are differences in the selection of patients, the tests used and the interpretation of the results. The aim of this review is to highlight the main problems found when measuring reversibility in such patients, the different factors that influence the reversibility testing and the biases which may confound the interpretation of the results.