This study explored reported barriers to treatment completion in a sample of adolescents and their parents who either completed or did not complete family-based cognitive behavioral lifestyle intervention for overweight and obese adolescents. The sample comprises 56 overweight or obese adolescents (52% female) aged 11.5-18.9 years (mean = 14.5, s.d. = 1.8) and a parent. 57% of families did not complete treatment and maintenance phases of the intervention. A telephone-administered questionnaire assessing barriers to participation was completed by 96% of adolescents and 91% of parent completers and 100% of adolescents and 94% of parent noncompleters. Adolescents and parents most commonly reported barriers to participation related to research demands, treatment approach, program components/strategies, practical barriers, and other individual/family demands. Parents also noted adolescent effort, parent-adolescent conflict, and adolescent unhappiness as barriers to participation. While both completers and noncompleters experienced barriers to participation, families who discontinued treatment reported experiencing more treatment barriers. Findings of the current study suggest that adolescents and parents may find it easier to participate in adolescent overweight and obesity interventions if research and out-of-session program demands are minimized, efforts are made to enhance adolescent motivation, and treatment is offered in a convenient location and scheduled around school holidays and other family demands. Results also suggest that targeting adolescent unhappiness, family stressors, and parent-adolescent conflict in treatment may improve retention. Future research should explore the impact of these modifications on treatment completion and outcomes.