The aim of this study was to evaluate the effectiveness of a computer-tailored physical activity intervention delivered through the Internet in a real-life setting. Healthy adults (n=526), recruited in six worksites, between 25 and 55 years of age were randomized to one of three conditions receiving, respectively, (i) online-tailored physical activity advice + stage-based reinforcement e-mails, (ii) online-tailored physical activity advice only, (iii) online non-tailored standard physical activity advice. At 6-month follow-up, no differences in physical activity between study conditions were found; total physical activity, physical activity at moderate intensity and physical activity in leisure time significantly increased in all study conditions between baseline and follow-up. Further evaluation of the intervention materials showed that the tailored advice was more read, printed and discussed with others than the standard advice. Most of the respondents in the e-mail group indicated to be satisfied about the number, frequency and usefulness of the stage-based e-mails. In conclusion, although tailored advice was appreciated more than standard advice, no evidence was found that an online-tailored physical activity intervention program outperformed online standard information.