Background: Wheeze-associated disorders are common in childhood, associated with considerable morbidity, if not detected and treated. Under diagnosis of asthma remains a problem, especially in resource-limited settings. Methods: We used a validated school-based screening questionnaire to detect children likely to have asthma. Children with positive screening were referred to the Pulmonology Department for clinical review and lung function testing. We compared asthma-like symptoms, activity limitation, school absence and health service utilization before and after in those diagnosed with, and treated for asthma. Results: 6400 children, from a potential population of 70,000 were screened between 2010 and 2016, with 900 (14.1%) screening positive. Lung function data were available from 578 (64.2%) children (5.7 to 6.5 years old). Asthma was confirmed in 549 children; 438 were treated with short acting bronchodilator alone and 111 with inhaled corticosteroids. Asthma control improved in 58% of children, with fewer daytime [mean 4.7 (SD1.9), vs 11.1 (0.6) days per week, p<0.001] and nocturnal [4.3 (1.1) vs 0.89 (0.5) days per month, p<0.001] symptoms. Activity improved and fewer school days were lost due to asthma [12.8 (3.0) vs 1.9 (0.9) days in past 3 months, p<0.001] in over 50% of children. Emergency department visits were reduced [1.8 (0.7) vs 0.3 (0.2) visits in past 3 months, p<0.001] in over 80% of children. Conclusions: Asthma under diagnosis remains a problem in Argentina. Our school-based assessment is an effective tool for detecting children with undiagnosed asthma. Instituting effective asthma treatment in these children reduces symptoms and improves control.