OBJECTIVE: To examine the responsiveness to change of the Spanish version of the Juniper Mini Asthma Quality of Life Questionnaire (Mini-AQLQ). METHODS:253 patients with mild to moderate uncontrolled asthma (patients with symptomatology and/or need for short-acting ƒ"2-agonists) were included in the study (61% women, mean age 36 years). A full history and physical examination were performed and montelukast was added at the baseline visit. All subjects completed the Mini-AQLQ questionnaire twice: at recruitment and after two months. Differences in patient scores before and after the montelukast addition were analysed using paired t-test. Responsiveness was assessed by calculating the standardized effect size (SES). A within-subject change in score of 0.5 is defined as the minimal clinically important difference (MCID). RESULTS: The Mini-AQLQ was responsive to changes over a two-month period. All Mini-AQLQ global and domain scores significantly improved after montelukast addition (p<0.01 for all comparisons). Mini-AQLQ score changes were significantly different for patients who improved and those that remained stable or deteriorated (p<0.001). The global score effect size was 0.91, ranging from 0.5 to 1.0 for the domains. The percentage of patients with mild and moderate asthma who were considered to have experienced a MCID in global score was 57.5% and 71.4% respectively, with average baseline scores of 5.0 and 4.3 respectively. The domain that experienced the greatest number of patients experiencing a clinically important improvement was Symptoms, with 65% and 78% of patients with mild and moderate asthma respectively. CONCLUSIONS: The Spanish version of the Mini-AQLQ is suitable for use in longitudinal studies where it is appropriate to assess the impact of asthma on the quality of life of individual patients with mild to moderate asthma. A high proportion of patients experienced a clinically meaningful improvement in their Quality of Life after addition of montelukast to their asthma therapy.