BACKGROUND: Although experts claim that computer-tailored interventions provided over the Internet have great potential to promote health behavior change, few studies have tested the efficacy of computer-tailored lifestyle interventions online-delivered over the Internet. PURPOSE: To evaluate the short-term (1 month) efficacy of an Internet-delivered, computer-tailored lifestyle intervention targeting saturated fat intake, physical activity (PA), and smoking cessation, and to evaluate exposure to the intervention. METHODS: A pretest-posttest randomized controlled trial with an intervention group and a no intervention waiting list control group was conducted. Self-reported behavior and determinants were assessed at baseline and 1 month follow-up. Exposure to the intervention was monitored through server registrations. The data were analyzed using multiple linear and logistic regression analysis. RESULTS: The intervention resulted in a significantly lower self-reported saturated fat intake (b = -0.76, p < 0.01) and a higher likelihood of meeting the PA guidelines among respondents who were insufficiently active at baseline (OR = 1.34, 95%CI = 1.001-1.80). No significant intervention effects were found for self-reported smoking status. Of the participants, 81% actually visited the website. CONCLUSIONS: The Internet-delivered, computer-tailored lifestyle intervention was effective in reducing self-reported saturated fat intake and in increasing self-reported PA among participants who completed the study.