The randomized controlled trial evaluated the effectiveness of an intervention based on the Transtheoretical Model of Change on anthropometric, metabolic and motivational outcomes in obese adolescents. A total of 135 male and female adolescents were randomized to two groups: intervention group (n = 65) and control group (n = 70). The adolescents were evaluated 1 week before the interventions began and at the end of 12 weeks. There was no statistically significant difference between groups in the outcome variables. Intervention group reported magnitude of effect more expressive on body mass index percentile, waist circumference, waist-to-hip ratio, readiness to change diet and readiness to start exercise.