This study evaluated the cost effectiveness of the "Power Breathing" program for asthma among middle and high school students. Few systematic evaluations of school based interventions--let alone cost-effectiveness programs--have been identified in the literature. Power Breathing was developed by the Asthma and Allergy Foundation of America and is currently available for implementation by school districts. For the overall evaluation, 8 junior high schools and 2 high schools were selected and matched based on grade range, enrollment, income and race/ethnicity. Schools were randomly assigned to the intervention or control group. Surveys were administered at baseline, immediately post-intervention and 3 months' post-intervention. Children in the intervention arm had a significant decrease in the number of days they experienced an asthma attack or had trouble breathing during a 2-week period of 0.18 days per 2 weeks. In contrast, subjects in the control group had an increase in the in the number of days experiencing an asthma attack or having trouble breathing during a 2-week period of 0.102, from 0.696 to 0.793. The program cost approximately $3.9 per asthma attack-free day gained, on par with pharmaceutical interventions, suggesting that Power Breathing may be a cost effective asthma intervention. The results of this study suggest that school-based interventions aimed at asthma, properly implemented and administrated, are an appropriate use of societal resources.