This study explores the impact of a cognitive behavioural lifestyle program, the Choose Health Program, on psychosocial wellbeing in overweight and obese adolescents. The sample comprised 29 male and 34 female adolescents aged 11.5 to 18.9 years (M = 14.3, SD = 1.9) and classified as overweight (n = 15) or obese (n = 48). Participants were randomly allocated to treatment or wait-list control conditions; participants allocated the wait-list condition were offered treatment after 6 months. Adolescents and parents completed self-report measures of psychopathology, psychosocial and family functioning. Treatment did not have detrimental effects on the psychosocial factors assessed. It resulted in significant improvements in weight control behaviour, impulse regulation, social support from family and parent–adolescent problem communication (p < .05). Similar results were obtained with completer and intention-to-treat analyses. Treatment acceptability was high, with all respondents indicating that they made progress. Combined, results indicate that treatment did not have detrimental effects on psychopathology, psychosocial functioning, or family functioning. Treatment resulted in significant improvements in impulse regulation, social support from family and parent–adolescent communication. Thus, parents and professionals can be assured that a comprehensive, multifaceted, parent-supported, cognitive behavioural intervention for overweight and obese adolescents does not cause psychological harm.