Background: The evaluation concerns a 12‐week group treatment programme providing formalized behaviour and cognitive modification combined with an initial low calorie liquid diet offered through a specialist NHS Obesity Clinic. Aims: The aims of this evaluation were to: (1) determine the success of the programme in terms of: (a) producing medically significant weight loss for up to 18 months after the programme; (b) psychological and social benefits for patients; (2) identify factors predictive of weight loss and the likelihood of completing the programme. Methods: A retrospective evaluation was undertaken of 12 groups of patients (115 patients), referred by their General Practitioners to the Obesity Clinic for advice and treatment of obesity. Data were obtained from initial assessment questionnaires, clinical notes and a follow‐up questionnaire. Results: The evaluation showed that the programme was successful in helping patients achieve medically significant weight loss (on average 12% of initial body weight) and to partially maintain some weight loss for up to 18 months after. In addition, there were measurable psychological and social benefits from their adherence to the programme. Only the mental state of the patients at the start of the programme was predictive of the likelihood of their managing to complete it. Conclusion: The progamme can be effective in achieving and maintaining weight loss. Patients may benefit psychologically. Patients may have an improved likelihood of completing the programme if depression is treated first.