Bronchial asthma often remains uncontrolled despite treatment with inhaled corticosteroids (ICS), long-acting β₂-agonists (LABA) or both, necessitating additional treatment. Patients ≥18years (n=1681) with mild-to-moderate asthma received oral montelukast 10mg added to ICS or ICS+LABAs, and were followed for 6months in a prospective, open-label observational study. The primary endpoint was change in Asthma Control Test (ACT) score. Secondary endpoints included mini-Asthma Quality-of-Life Questionnaire (mini-AQLQ) and FEV₁/PEF. Mean ACT scores improved from 14.6±4.6 (baseline) to 19.4±4.4 (month 6; p<0.0001). Using ACT score categories, the percentage of patients with uncontrolled (57.5%) or poorly controlled (25.0%) asthma at baseline decreased at month 6 (17.6 and 21.7%, respectively); the percentage of patients with well controlled (13.9%) or completely controlled (1.2%) asthma at baseline increased at month 6 (47.5 and 11.4%, respectively). The mini-AQLQ score (mean±SD) improved from 4.0±1.1 to 5.3±1.1 (p<0.0001); FEV₁ increased from 2.46±0.89 to 2.60±0.92L (p<0.0001). Treatment with montelukast was generally well tolerated. In patients insufficiently controlled with ICS or ICS+LABAs, daily add-on montelukast improved both asthma control and asthma-related quality of life. Clinicaltrials.gov registry number NCT00802789.