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Motivational interviewing (MI) is an effective method to promote weight loss that can be delivered by non-mental health providers. The aim of this study was to evaluate whether MI was superior to conventional counseling to improve the anthropometric outcomes of adolescents with obesity/overweight. It was a controlled cluster randomized trial with parallel design in a school setting. The study included two groups: Motivational Interviewing Group (MIG) and control group (Conventional Intervention Group, CIG). Students participated in three face-to-face 30-min interviews, 3 months apart. Outcomes were BMI z-score, abdominal circumference, percentages of fat mass and muscle mass, and blood pressure. Sessions were coded with the Motivational Interviewing Treatment Integrity (MITI) manual. Mixed repeated-measures ANOVAs were used to assess the group versus time interaction. Effect sizes were calculated for each ANOVA with eta-squared measures (η 2 ). Eighty-three adolescents finished the protocol. While MIG participants showed a significant improvement in all anthropometric scores at 6 months, CIG participants showed an unfavorable change in those variables.Conclusion: Our results provide additional evidence of the short-term usefulness of a school-based MI intervention on anthropometric outcomes of adolescents with obesity/overweight, demonstrating that pediatricians can play an important role in the prevention and management of pediatric obesity.Trial registration: The study is called IMAGINE and is registered in Clinicaltrials.gov with the number NCT02745795. What is Known: • Although MI has been recognized as an effective counseling style for behavioral change in weight loss, there are few reports about the anthropometric outcomes of interventions with adolescents being treated for obesity/overweight. • Our study showed significant positive changes in anthropometric variables (BMI z-score, abdominal circumference, percentage of fat mass, percentage of muscular mass, systolic and diastolic blood pressure) after only three face-to-face sessions over 6 months. What is New: • MI delivered by non-mental health providers in a school setting seems to have short-term usefulness in a program aiming the treatment of obese/overweight adolescents.