The aims of the present study were to investigate (1) the proportion of adolescents in each of the stages of change, (2) the differences in psychosocial factors and in physical activity between the stages, and (3) the classification accuracy using several reference criteria. A random sample of 38 schools from the Flemish community in Belgium resulted in a sample of 5931 adolescents (mean age=14.8, range 12-18, 61% females). All adolescents completed a computerized questionnaire assessing demographic variables, physical activity, physical activity determinants and stages of change. Distribution across the stages was: precontemplation, n=684 (11.5%); contemplation, n=948 (16.0%); preparation, n=818 (13.8%); action, n=492 (8.3%); and maintenance, n=2989 (50.4%). Analyses revealed gender and age differences (P<0.001), with girls and older adolescents being more prevalent in the first three stages. Differences between the stages of change were related to higher levels of physical activity of different intensity and within different contexts (P<0.001), together with more favorable psychosocial determinants related to physical activity (P<0.001). No differences between stages were found for sedentary activities. Three subgroups of adolescents were identified based on psychosocial determinants and physical activity levels. About 71% of the adolescents placed themselves in a stage which was in accordance with their self-reported level of physical activity.