Objective: To determine factors which predict non-completion of group nondieting interventions for overweight women, and to investigate whether completion improves outcomes. Design: First, baseline predictors of non-completion were identified; then changes at 10 weeks and 12 months were compared between completers and non-completers of 10-week non-dieting interventions. Setting: General community. Subjects: Participants were 119 women (aged 25-65 years, BMI $ 28 kg/m 2 ) with at least one cardiovascular risk factor. Participants who attended at least eight of the ten sessions were classified as completers, and non-completers were those who attended fewer than eight sessions. Measures included BMI, blood pressure, psychological distress, lifestyle behaviours and eating self-efficacy. Results: Logistic regression analyses indicated that women were less likely to be noncompleters at non-dieting group programmes if, at baseline, they were more highly educated or had healthier nutrition behaviours (controlling for education). Only healthier nutrition behaviour was negatively associated with non-completion in the final model. Twelve months after the intervention, completers showed significantly greater improvements in body weight (mean change 20?53 kg), systolic and diastolic blood pressure (26?3 and 24?1 mmHg, respectively), stress management behaviour score (10?5) and psychotic symptoms score (20?1) than non-completers (all P , 0?05). Conclusions: Highly educated women already engaging in some healthier lifestyle choices were less likely to be non-completers in non-dieting group programmes. Since important treatment outcomes vary according to attendance, future trials of non-dieting interventions should report the effects of completion on outcomes.