Mild persistent asthma is most effectively controlled with inhaled corticosteroids. Leukotriene receptor antagonists have complementary effects to corticosteroids on inflammation control. The additional effect of a leukotriene receptor antagonist, zafirlukast, was investigated in stable asthma patients under control with inhaled budesonide. We conducted a randomised, double-blind, placebo-controlled, single center trial to investigate the effects of add-on zafirlukast treatment to budesonide, on symptom score, pulmonary function, bronchial responsiveness, and serum levels of eosinophilic cationic protein (ECP) and antioxidant capacity in stable asthmatic patients under control with inhaled budesonide. The present study included 21 mild or moderate asthmatic patients (8 males and 13 females), who were stable at least for 6 weeks with inhaled budesonide (400 μg/day). Serum total antioxidant capacity (TAC) and ECP levels were measured, and symptom scoring, spirometry, and bronchial provocation with methacholine were performed. Then, the patients were randomised to use either placebo or oral zafirlukast (40 mg/day) in addition to budesonide for 6 weeks. At the 6th week, symptom scoring, spirometry, and bronchial provocation tests were repeated and serum TAC and ECP levels were measured again. After add-on zafirlukast treatment to budesonide, forced expiratory volume in 1 second (FEV₁), TAC and ECP values did not change significantly (p > 0.05) but bronchial hyperresponsiveness and symptom score decreased significantly (p = 0.022) compared to baseline. Thus, in stable asthmatic patients, add-on zafirlukast treatment to budesonide improves symptoms and decreases bronchial hyperresponsiveness.