Recently, we published data of a 2 year randomized controlled study in which the effects of continuous versus symptomatic bronchodilator treatment in patients with moderate asthma or chronic bronchitis were investigated. The results showed that FEV1 decline in the continuously treated group was significantly larger than in the symptomatically treated group (72 versus 20 ml/year, P < 0.05). We reanalysed these data in order to investigate the hypothesis that the continuous use of bronchodilators may mask a rapid decline in lung function. Lung function decline was assessed by regression analysis of seven FEV1 measurements. Respiratory symptoms were assessed by means of the MRC questionnaire every 12 months, and they were also recorded by the patients in a weekly report. Of the participating patients 144 completed the study. Increased lung function decline in the continuously treated group was not reflected in a significant deterioration of the symptoms. Moreover, the decline in FEV1 showed no correlation at all with changes in respiratory symptoms in continuously treated patients (r = -0.03, P = 0.80), whereas in the symptomatically treated group, there was a better relation (r = -0.32, P = 0.003) to changes in respiratory symptoms. These results show that continuous bronchodilation may indeed mask the worsening of the disease. This lack of awareness of deterioration of the disease is probably caused by the continuous symptom relief of bronchodilators. It may be misleading to both patients and physicians.