Objectives: We assessed policy and system changes and health outcomes produced by the Allies Against Asthma program, a 5-year collaborative effort by 7 community coalitions to address childhood asthma. We also explored associations between community engagement and outcomes. Methods: We interviewed a sample of 1,477 parents of children with asthma in coalition target areas and comparison areas at baseline and 1 year to assess quality-of-life and symptom changes. An extensive tracking and documentation procedure and a survey of 284 participating individuals and organizations were used to ascertain policy and system changes and community engagement levels. Results: A total of 89 policy and system changes were achieved, ranging from changes in interinstitutional and intrainstitutional practices to statewide legislation. Allies children experienced fewer daytime (P = 0.008) and nighttime (P = 0.004) asthma symptoms than comparison children. In addition, Allies parents felt less helpless, frightened, and angry (P = 0.01) about their child's asthma. Type of community engagement was associated with number of policy and system changes. Conclusions: Community coalitions can successfully achieve asthma policy and system changes and improve health outcomes. Increased core and ongoing community stakeholder participation rather than a higher overall number of participants was associated with more change.