The increasing prevalence of obesity has been mirrored by a parallel increase in the number of commercial weight loss programmes. Research evaluating these programmes is meagre, however, compared to the numbers treated. Reluctance of commercial weight loss programmes to meaningfully evaluate their weight loss efficacy may arise from fear that competitors will use the results against them. Evaluation of commercial weight loss programmes usually progresses from testimonials, often by famous people who were successful, to uncontrolled studies of past participants evaluated either by the programme itself or by an outside entity. The gold standard, however, is a scientifically rigorous, controlled study of the programme conducted by an independent entity. Such a study, published in a peer-reviewed journal, can gain credibility for a programme, as it did with Slim Fast, if the results are positive, or herald the end of the programme, as it was with Simeons human chorionic gonadotropin injection clinics. This review of the evolution of the evaluation process of commercial weight loss programmes leads us to conclude that consumers are likely to demand greater scientific rigour in the future, a change that will favour informed choice and discourage the practice of unrealistic advertising that raises false hopes.