In asthmatic patients, antioxidant defence is decreased. Although inhaled corticosteroids decrease asthmatic inflammation and modulate reactive oxygen species (ROS) generation, little is known of their effect on cellular antioxidant levels. The aim of this study was to evaluate the effect of inhaled beclomethasone dipropionate (BDP; 1,000 microg x day(-1)) on erythrocyte antioxidant levels in stable asthmatic patients. Forty patients with stable, mild asthma were treated in a double-blind, placebo-controlled, parallel-group study with BDP 250 microg, two puffs b.i.d. for 6 weeks. At entry and every 2 weeks during treatment, erythrocyte antioxidant levels, haematological parameters, pulmonary function tests and asthma symptoms were determined. The results show that during treatment with BDP, erythrocyte catalase levels increased (at entry (mean +/-SEM) 41+/-4, after 6 weeks 54+/-4 micromol H2O2 x min(-1) x g haemoglobin (Hb)(-1), p = 0.05 in comparison with placebo). Erythrocyte total glutathione levels significantly decreased after 6 weeks treatment with BDP (from 7.0+/-0.4 to 6.6+/-0.3 micromol x g Hb(-1) (p = 0.04)). In the BDP-treated patients, blood eosinophil counts were higher in patients who responded with an increase in erythrocyte catalase levels during BDP treatment, as compared to those not responding ((mean +/-SEM) 340+/-39 and 153+/-52x10(6) cells x L(-1), respectively, p = 0.05). The present study shows that treatment with inhaled bedomethasone dipropionate results in changes in antioxidant levels in erythrocytes of patients with stable, mild asthma.