BACKGROUND: Undue influence of body shape or weight on self-evaluation - referred to as overvaluation - is considered a core feature across eating disorders, but is not a diagnostic requirement for binge eating disorder (BED). This study examined the concurrent and predictive significance of overvaluation of shape/weight in obese patients with BED participating in a randomized clinical trial testing cognitive behavioral therapy (CBT) and behavioral weight loss (BWL). Method A total of 90 participants were randomly assigned to 6-month group treatments of CBT or BWL. Assessments were performed at baseline, throughout- and post-treatment, and at 6- and 12-month follow-ups after completing treatments with reliably administered semi-structured interviews and established measures. RESULTS: Participants categorized with overvaluation (n = 52, 58%) versus without overvaluation (n = 38, 42%) did not differ significantly in demographic features (age, gender and ethnicity), psychiatric co-morbidity, body mass index or binge eating frequency. The overvaluation group had significantly greater levels of eating disorder psychopathology and poorer psychological functioning (higher depression and lower self-esteem) than the non-overvaluation group. Overvaluation of shape/weight significantly predicted non-remission from binge eating and higher frequency of binge eating at the 12-month follow-up, even after adjusting for group differences in depression and self-esteem levels. CONCLUSIONS: Our findings suggest that overvaluation does not simply reflect concern commensurate with being obese or more frequent binge eating, but also is strongly associated with heightened eating-related psychopathology and psychological distress, and has negative prognostic significance for longer-term treatment outcomes. Overvaluation of shape/weight warrants consideration as a diagnostic specifier for BED as it provides important information about severity and treatment outcome.