Background: Helping young smokers to quit early in life substantially reduces the risk of later morbidity and mortality due to tobacco use. The RealU study demonstrated the efficacy of a smoking-cessation website for college students that incorporated both individually tailored feedback and peer e-mail support. The relationship between peer e-mail support and cessation outcomes among intervention participants is examined here. Methods: This study was conducted at the University of Minnesota Twin Cities from Fall 2004 through Spring 2005. During the intervention period, peer-support students (E-pals) wrote weekly e-mails to intervention group participants (n=257) encouraging healthy behaviors including smoking abstinence. Ten survey items assessed perceived E-pal supportiveness. The number of e-mails replies sent by the participants to their E-pal was tracked as a measure of e-mail engagement. The primary outcome was self-reported 30-day abstinence at the end of the intervention period. Results: Over the course of the intervention, participants sent an average of 4.6 (SD=3.6) e-mails to their E-pals. Perceived E-pal support was significantly correlated with e-mail engagement (p<0.001). At Week 30, 40.5% of individuals in the RealU intervention group (104/257) reported not smoking any cigarettes in the prior 30 days. Bivariate analyses indicated that 30-day abstinence was related to both perceived support from the E-pal (p<0.001) and e-mail engagement (p<0.001). Multivariate analyses indicated that after controlling for age and baseline-level smoking, e-mail engagement remained a significant predictor of 30-day abstinence (p<0.001). Conclusions: Greater peer engagement via e-mail was associated with increased smoking abstinence and reduced frequency of smoking. These findings suggest that online peer support may be an important strategy when delivering Internet-assisted cessation programs to young adults.