Secondhand tobacco smoke (SHS) exposure increases asthma morbidity in children. Efforts to reduce exposure have had mixed results. This study is a randomized controlled trial of an exposure reduction intervention, with objective feedback to parents on the child's exposure based on urine cotinine measurement, and counseling tailored to the child's specific exposure sources/locations and parental readiness to take specific actions to reduce exposure from each source/location. This trial involves 350 SHS-exposed children with persistent asthma, 3-12 years of age, receiving care from the Kaiser Health Care Program in Northern California. Primary outcomes over the 18 months of follow-up will be asthma acute care utilization and urine cotinine/creatine ratio. Changes in controller medication adherence will be evaluated using a pharmacy-based dispensing index.