© 2015 The Authors. Published by Elsevier Ireland Ltd. This is an open access article under the CCBY-NC-ND license. Objective This study examined the effects of a non-dieting lifestyle intervention approach for morbidly obese women designed in the framework of the self-determination theory (SDT) and Health at Every Size on weight maintenance and psychological functioning. Participants and design Predominantly white (97%), morbidly obese (BMI ≥ 35 kg m -2 with at least one co-morbid condition or a BMI ≥ 40 kg m -2 ) pre-menopausal women (N = 62), aged between 24 and 55 years were initially randomly assigned to 12 weeks of lifestyle intervention (IIG) or delayed start control group (DSCG). The program consisted of 3 months intensive lifestyle intervention followed by 9 month maintenance phase. The DSCG group commenced the program after 3 months. Results and conclusions Initially, the IIG showed a significant decrease in body weight (baseline to end of the RCT phase) compared with a significant increase in the DSCG group. However, no significant changes in weight status were evident in either group at 12 months compared with baseline. The 3-month intensive intervention resulted in significantly improved psychological functioning in both groups, which were maintained at 12 months. The study provides additional support for a non-dieting, theory-based, lifestyle approach to weight management and psychological well-being among morbidly obese females.