AIM: This study evaluates the efficacy of the Choose Health program, a family-based cognitive behavioural lifestyle program targeting improved eating and activity habits, in improving body composition, cardiovascular fitness, eating and activity behaviours in overweight and obese adolescents. METHOD: The sample comprised 29 male and 34 female overweight (n = 15) or obese (n = 48) adolescents aged 11.5-18.9 years (M = 14.3, SD = 1.9). Participants were randomly allocated to treatment or waitlist control conditions; waitlist condition participants were offered treatment after 6 months. DEXA-derived and anthropometric measures of body composition; laboratory-based cycle ergometer and field-assessed cardiovascular fitness data; objective and self-report physical activity measures; and self-report measures of eating habits and 7-day weighed food diaries were used to assess treatment outcome. Adherence to treatment protocols was high. RESULTS: Treatment resulted in significant (p < .05) and sustained improvements in a range of body composition (body fat, percent body fat, lean mass) and anthropometric measures (weight, BMI, BMI-for-age z-score and percentiles). Minimal improvements were seen in cardiovascular fitness. Similar results were obtained in completer and intention-to-treat analysis. Poor adherence to assessment protocols limits conclusions that can be drawn from physical activity and dietary data. CONCLUSIONS: Participation in the Choose Health program resulted in significant improvement in body composition. Longer-term follow up is required to determine the durability of intervention effects. Alternative approaches to the measurement of diet and physical activity may be required for adolescents.