To determine factors which predict non-completion of group non-dieting interventions for overweight women, and to investigate whether completion improves outcomes.

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.

General community.

Participants were 119 women (aged 25–65 years, BMI ≥ 28 kg/m2) 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.

Logistic regression analyses indicated that women were less likely to be non-completers 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 −0·53 kg), systolic and diastolic blood pressure (−6·3 and −4·1 mmHg, respectively), stress management behaviour score (+0·5) and psychotic symptoms score (−0·1) than non-completers (all P < 0·05).

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.