To date, few Internet-delivered smoking cessation interventions have been tested. This study tested the efficacy, understandability, credibility and personal relevance of an e-mail-delivered computer-tailored smoking cessation intervention. It included tailored action plan feedback, as recent studies have demonstrated the importance of planning in facilitating quitting smoking. Participants (Dutch adults) were randomly assigned to the intervention (computer-tailored e-mail; N = 224) or the control group (generic, non-tailored e-mail; N = 234). The results 6 months after baseline (N = 195) showed that significantly more participants in the intervention group reported not having smoked in the last 24 hours (21.5%) and 7 days (20.4%) in contrast with participants in the control group (9.8 and 7.8%, respectively). Intention-to-treat analyses revealed similar results, though overall lower quitting percentages. Furthermore, participants in the intervention group appreciated the computer-tailored e-mail significantly more in terms of understandability, credibility and personal relevance. Hence, the computer-tailored intervention is effective for the Dutch smoking population motivated to quit smoking. Further research is needed into the efficacy of the intervention for smokers who are not motivated to quit smoking and into the benefits of (multiple) e-mail-delivered tailored letters with tailored action plan feedback over and above tailoring without action plan feedback.