PURPOSE Guideline implementation in primary care has proven difficult. Although external assistance through performance feedback, academic detailing, practice facilitation (PF), and learning collaboratives seems to help, the best combination of interventions has not been determined. METHODS In a cluster randomized trial, we compared the independent and combined effectiveness of PF and local learning collaboratives (LLCs), combined with performance feedback and academic detailing, with performance feedback and academic detailing alone on implementation of the National Heart, Lung and Blood Institute's Asthma Guidelines. The study was conducted in 3 primary care practice-based research networks. Medical records of patients with asthma seen during pre- and postintervention periods were abstracted to determine adherence to 6 guideline recommendations. McNemar's test and multivariate modeling were used to evaluate the impact of the interventions. RESULTS Across 43 practices, 1,016 patients met inclusion criteria. Overall, adherence to all 6 recommendations increased (P