Asthma is one of the most common chronic diseases in children and adults. Recent studies have shown that in asthmatic patients treated with inhaled corticosteroids there is a better diseases control when adding a second drug, than increasing the corticosteroids dose. The aim of this study has been to evaluate the effectiveness and tolerance of zafirlukast, a leukotriene receptor antagonist, versus budesonide in clinically steady patients with mild persistent bronchial asthma. We have enrolled 36 subjects non smokers, with mild persistent bronchial asthma and 12 healthy subjects as control group. At the beginning of this study and at the end of the treatment (8 weeks), all patients underwent complete clinical work-up, pulmonary function testing (FEV1, PEF and FVC) and methacholine challenge test. The patients were divided into 3 groups: group A) 20 mg of zafirlukast twice a day; group B) 400 mg of budesonide twice a day; group C) 20 mg of zafirlukast twice a day and 400 mg of budesonide twice a day. Basal FEV1 and PEF presented no significant statistical differences between control subjects and patients of group A, B and C. After eight weeks there were no significant changes for FEV1 and PEF among the three groups. After therapy a strong significant increase of PD20 was documented in group A (p<0.005), group B (p<0.001) and group C (p<0.005), respect to baseline values. The antileukotriene drugs could be taken as an alternative drug, or in association with low-dose inhaled corticosteroids, in patients with mild persistent asthma, both for their clinical effectiveness and their easy ingestion, which is confirmed in compliance studies on inhaled steroids.