Although group-based programs remain the most common treatment approach for adolescent smoking cessation, success rates for these programs have been relatively modest, and their reach may be limited. Web-based adjuncts may be one way to boost the efficacy and reach of group-based approaches. The purpose of this study was to evaluate the effectiveness of enhancing the American Lung Association's Not on Tobacco program (NOT) with a Web-based adjunct (NOT Plus). Twentynine high schools were randomly assigned to either the NOT program alone or to the NOT Plus condition, which included access to a specially designed Web site for teens, along with proactive phone calls from the group facilitator to the participant. Self-reported smoking behavior was obtained at end-of-program and at a 3-month follow-up. Using hierarchical linear modeling, accounting for the clustering of students in schools, and controlling for student gender, grade, race, and baseline smoking rate, there was a marginally significant (p = .06) condition effect at end-of-treatment and a significant effect at 3-month follow-up (p < .05) favoring the NOT Plus condition. Approximately 57% of adolescents reported visiting the Web site, and among the NOT Plus condition, use of the Web site was associated with cessation significantly at end-of-program (p < .05), but not at 3 months. Adolescents in urban schools were more likely to access the Web site than those in rural schools. Participants who visited the Web site rated it positively on several dimensions. Reasons for not using the Web site will be discussed, as well as its value as an adjunct.