To test the hypothesis that inhaled glucocorticoids affect the level of CYP3A5 mRNA of alveolar macrophages in patients with respiratory diseases in vivo, CYP3A5 mRNA was measured with quantitative RT-PCR. The differences of CYP3A5 expression in current glucocorticoidusers, ex-users, and non-users were minimal and statistically non-significant (P=0.809). The use of glucocorticoids elevated the CYP3A5 level by 45% in non-smokers (P=0.438). However, the patients’ smoking status had profound consequences for the CYP3A5 levels. CYP3A5 expression in current smokers was only 7 % of the CYP3A5 level in non-smokers (P < 0.05). In ex-smokers the expression was slightly reduced (88 %) compared to non-smokers (P < 0.106). Sex and age were tested as confounding factors, but they did not influence the results.