BACKGROUND Long-acting-beta-agonists and inhaled corticosteroids are widely used in patients with obstructive lung disease. We determined the occurrence of potential side effects to inhaled steroids and long-acting-beta-agonists in an open post-marketing observational study. METHODS A total of 158 adult patients treated with long-acting-beta-agonists and inhaled steroids because of asthma or chronic obstructive lung disease were included prospectively in a cross-sectional study from September 2004 through august 2005. A subgroup of 31 patients with minimal disease were observed prospectively after treatment reduction. The frequency and intensity of potential side effects to long-acting-beta-agonists and inhaled corticosteroids were registered on a 10-point visual analogue scale. RESULTS A total of 131 (83%) reported potential side effects to inhaled steroids. The most frequent were sore (54%) and dry (52%) throat. A total of 114 (72%) patients reported potential side effects to long-acting-beta-agonists. The most frequent were muscle cramps (62%) and muscle twisting (39%). For 5/8 potential side effects their occurrence increased with increased individual dozing. In 31 patients the mean medication with steroids and long-acting-beta-agonists was reduced by 87% and 97.5%, respectively, with side effects reduced by 62% (p<0.001) to inhaled steroids and by 91% (p<0.001) to inhaled long-acting-beta-agonists. CONCLUSIONS Potential side effects in adult patients to inhaled corticosteroids and long-acting-beta-agonists are very common with increased frequency with increased dozing. Patients are largely unaware of the association. Patients should be better informed, and the results further support tailored dozing to minimum therapy.