Objective. To examine if a website-delivered physical activity intervention, that provides participants with computer-tailored feedback, can improve physical activity in the general population. Methods. Healthy adults (n=434), recruited from parents and staff of 14 primary and secondary schools in Belgium in the spring of 2005, were allocated into one of two intervention groups (receiving intervention with or without repeated feedback) or a no-intervention control group. Physical activity-levels were self-reported at baseline and at 6 months (n=285), using a computerized long version of the International Physical Activity Questionnaire online. Repeated measures analysis of co-variances were used to examine differences between the three groups. Results. Intent-to-treat analysis showed significant time by group interaction effects in favor of both intervention groups compared with the control group. Significant increases were found for active transportation (+20, +24, +11 min/week respectively) and leisure-time physical activity (+26, +19, −4 min/week respectively); a significant decrease for minutes sitting on weekdays (−22, −34, +4 min/day respectively). No significant differences were found between both intervention groups. Conclusion. A website-delivered intervention, including computer-tailoring, was able to increase physical activity when compared to a no-interventioncontrol group. High drop-out rate and the low number of participants who received repeated feedback indicated that engagement and retention are important challenges in e-health studies.