The purpose of this study was to describe changes in physical activity and determinants in youngsters treated for obesity. Self-reported physical activity and determinants were assessed at start, during and after an intervention in 24 obese patients (mean age 13.5+/-2.1 years). Participation in activities of moderate to high intensity increased during treatment, but 6-months after treatment this decreased again to pre-treatment levels. Participation in activities of low intensity and television watching decreased during treatment and increased following treatment. Very little change was found in perceived benefits, barriers or self-efficacy scores. Modelling of siblings decreased after treatment, whereas there was no evolution in modelling of parents. Social support by family members increased during treatment, but decreased following treatment. We can conclude that the intervention was successful in activating obese youngsters to move at the desirable exercise intensity and frequency; however, maintenance of the exercise habit after treatment was not successful.