Weight loss is difficult to achieve and predict. We used Carver & Scheier's (1998) self-regulation theory to investigate the role of dispositional and situated optimism in weight loss following a group intervention. The theory proposes that dispositional optimism is related to persistence in goal pursuit as a result of greater confidence in goal attainment. Findings showed that situated optimism (higher goals, greater confidence in their attainment) predicted greater weight loss; whereas dispositional optimism, perceptions of controllability, timeline, and consequences of the weight problem were unrelated to weight loss. Changes in situated optimism following the intervention were related to weight-loss success/failure. The findings suggest that people possess quite accurate knowledge of their chances of success, which could be used to tailor interventions to participants.