Objective: The treatment of obesity is universally disappointing; although usually some weight loss is reported directly after treatment, eventual relapse to, or even above, former body weight is common. In this study it is tested whether the addition of cognitive therapy to a standard dietetic treatment for obesity might prevent relapse. It is argued that the addition of cognitive therapy might not only be effective in reducing weight and related concerns, depressed mood, and low self-esteem, but also has an enduring effect that lasts beyond the end of treatment. Methods: Non-eating-disordered overweight and obese participants in a community health center (N=204) were randomly assigned to a group dietetic treatment+cognitive therapy or a group dietetic treatment+physical exercise. Results: Both treatments were quite successful and led to significant decreases in BMI, specific eating psychopathology (binge eating, weight-, shape-, and eating concerns) and general psychopathology (depression, low sell-esteem). In the long run, however, the cognitive dietetic treatment was significantly better than the exercise dietetic treatment; participants in the cognitive dietetic treatment maintained all their weight loss, whereas participants in the physical exercise dietetic treatment regained part (25%) of their lost weight. Conclusion: Cognitive therapy had enduring effects that lasted beyond the end of treatment. This potential prophylactic effect of cognitive therapy is promising; it might be a new strategy to combat the global epidemic of obesity.