OBJECTIVES: We tested a Public Health Service 5As-based clinician-delivered smoking cessation counseling intervention with adolescent smokers in pediatric primary care practice. METHODS: We enrolled clinicians from 120 practices and recruited youth (age > /=14) from the American Academy of Pediatrics Pediatric Research in Office Settings practice-based research network. Practices were randomly assigned to training in smoking cessation (intervention) or social media counseling (attentional control). Youth recruited during clinical visits completed confidential screening forms. All self-reported smokers and a random sample of nonsmokers were offered enrollment and interviewed by phone at 4 to 6 weeks, 6 months, and 12 months after visits. Measures included adolescents' report of clinicians' delivery of screening and counseling, current tobacco use, and cessation behaviors and intentions. Analysis assessed receipt of screening and counseling, predictors of receiving 5As counseling, and effects of interventions on smoking behaviors and cessation at 6 and 12 months. RESULTS: Clinicians trained in the 5As intervention delivered more screening (beta = 1.0605, P < .0001) and counseling (beta = 0.4354, P < .0001). In both arms, clinicians more often screened smokers than nonsmokers. At 6 months, study arm was not significantly associated with successful cessation; however, smokers in the 5As group were more likely to have quit at 12 months. Addicted smokers more often were counseled, regardless of study arm, but were less likely to successfully quit smoking. CONCLUSIONS: Adolescent smokers whose clinicians were trained in 5As were more likely to receive smoking screening and counseling than controls, but the ability of this intervention to help adolescents quit smoking was limited.