The aim of the present study was to evaluate the effects of a middle school physical activity and healthy eating intervention, including an environmental and computer-tailored component, and to investigate the effects of parental involvement. A random sample of 15 schools with seventh and eight graders was randomly assigned to one of three conditions: (i) intervention with parental involvement, (ii) intervention alone and (iii) control group. In 10 schools, an intervention, combining environmental changes with computer-tailored feedback, was implemented over 2 school years. In five intervention schools, increased parental support was added. Physical activity was measured with questionnaires in the total sample and with accelerometers in a sub-sample of children. Fat intake, fruit, water and soft drink consumption were measured using food-frequency questionnaires. Results showed significant positive intervention effects on physical activity in both genders and on fat intake in girls. Parental involvement did not increase intervention effects. It can be concluded that physical activity and eating behaviours of middle school children can be improved by school-based strategies combining environmental and personal interventions. The use of personalized computer-tailored interventions seems to be a promising tool for targeting adolescents but needs to be further explored. (Contains 3 tables.)