Objectives: 23 percent of high school seniors are now smoking regularly, but our ability to promote cessation among teens is limited. Cessation programs have often had limited success in recruiting adolescents, and many youth programs have failed to demonstrate effectiveness. New models that are better adapted to young smokers are clearly needed. We have developed and pilot tested a promising program using peer counselors to deliver brief, tailored smoking cessation messages. The purpose of this study is to demonstrate the effectiveness of this intervention in assisting teens to quit. Methods: The study design is a randomized controlled trial of peer smoking cessation counseling. 16 schools will be randomized to either the intervention or control group. 265 high school students from eight schools will be trained as peer counselors. They will learn to assess smokers' stage of readiness to change, how to deliver semi-structured quit advice based on that stage, and methods for actively supporting a quit attempt. Counselors will recruit several of their smoking friends (450 total) and deliver two personalized cessation messages. The initial counseling session lasts 10-15 minutes with a 5 minute booster two weeks later. In eight control schools, 265 untrained "counselors" will recruit control smokers but will not provide advice. Intervention and control subjects will be compared at one, three and six months post-counseling in terms of: prolonged and 7-day point prevalence abstinence, number and duration of quit attempts, cigarette consumption, nicotine dependence, stage of readiness-to-change, intention to quit and attitudes about quitting. Salivary cotinine will confirm self-reported abstinence. Secondary effects of the intervention on the peers' friendship and on the counselors' smoking initiation will also be explored. In the analysis, hierarchical linear modeling will allow us to control for data that is clustered by counselor and school. A rigorous intention to treat analysis will be performed. Benefit: This new intervention is an innovative blend of transtheoretical and social influence theory constructs applied in a unique peer group setting. This peer counseling program shows the potential to help large numbers of teens in a resource efficient manner.