Montelukast is recognised to be an effective and safe therapy for the treatment of mild to moderate asthmatic patients with concomitant seasonal allergic rhinitis. OBJECTIVES: To evaluate asthma-related direct costs in patients with mild to moderate persistent asthma and seasonal allergic rhinitis (AR) in Spain, whose asthma was inadequately controlled and required the addition of montelukast as part of their routine care. METHODS: A multicenter, 12 months pre-post observational study was conducted, selecting mild to moderate asthmatic consecutive patients with previous inhaled corticosteroid (ICS) w/wo LABAs therapy, who initiated concomitant montekulast between January 1999-December 2002. Asthma-related health care resources data was collected retrospectively, including medication, medical visits, ER visits and hospitalizations. For the costing calculations, natural units were multiplied by unit costs, and confidence intervals (CI) calculated using bootstrapping analysis. RESULTS: A total of 212 patients (mean age 36.0(SD 9.7), 56.6% female, 50.9% mild asthmatics) were recruited in 55 sites (54.5% Primary Care, 34.5% Allergologists and 11% Pneumologists) across the country. After initiation of montelukast therapy, all other asthma-related healthcare resource categories did show a significant reduction (p<0.01) (mean reduction in 2004€; 95% CIs): medication (146.1; 78.8-225.2); outpatient visits (57.5; 44.5-76.8), ER visits (61.1; 48.4-84.4) and hospitalizations (243; 86.2-875.4), total 508.3 95% CI 348.3-969.9. Additionally considering the montelukast treatment cost during 365 days, the reduction achieved in all other asthma-related healthcare resources would compensate for 92.7% of the montelukast cost. CONCLUSIONS: Montelukast therapy was associated with a significant reduction in all other asthma-related health care resources use in patients with mild to moderate asthma and concomitant seasonal allergic rhinitis.