Aims:This study evaluated the effect of behaviour change interventions at Norwegian Healthy Life Centres on change in body mass index (BMI) and body attitude, and explored the predictors for change after 6 months. Methods: We randomised 118 participants to either an intervention or a control group. Eligible participants: ⩾18 years and able to take part in group-based interventions. Body attitude, weight, and height were assessed at inclusion and after 6 months. We analysed the data using simple and multiple regression. Results: Eighty-six participants completed 6-month follow-up. The study found no intervention effect on BMI or body attitude across the two groups. However, an interaction effect indicated that the leaner participants in the intervention group reduced their weight significantly ( b 0.94, p < 0.001). BMI reduction was predicted by self-efficacy for physical activity and autonomous motivation for change. Weight loss was associated with impaired body attitude, body shape concern, impaired weight-related self-esteem, weight cycling, and controlled motivation for change. Improvement in body attitude was positively impacted by self-rated health, the experience of childhood respect, life satisfaction, and self-efficacy for physical activity. Impaired body attitude was predicted by body shape concern, impaired weight-related self-esteem, and controlled motivation. Conclusions: The interventions did not affect body mass on average, but promoted weight loss among the leaner participants. Because weight reduction was associated body shape concern and impaired body attitude, the study supports the claim that interventions should be weight neutral and aim to improve body image and psychological well-being rather than weight reduction.