The aim of this study was to determine the efficacy and safety of montelukast added to previous medication in the treatment of a mild and moderate asthma. Data were obtained via questionnaires given to the physicians and given further to their patients. Patients were divided in two groups, first followed 4 weeks (612 patients) and second followed 8 weeks (91 patients). We found out that there was a significant improvement in FEV1 (forced expiratory volume in first second) and general condition of patients and decreased number of salbutamol inhalations after using montelukast. In the second group of patients we find out the same significant improvement in FEV1, general condition and decrease in salbutamol inhalations after 4 weeks of using montelukast and further improvement after the next month of therapy. We conclude that montelukast is an efficient drug with little side effects and with a good compliance. Montelukast managed to achieve a good asthma control; therefore it has a significant place in asthma therapy.