The objective of this study was to examine whether beliefs about the cause, consequences, time line, and control of obesity are predictors of the amount of weight loss after an 8-week, low-calorie diet consisting of meal replacements. Forty-eight women and 18 men, mean age=45.9 (range=23 to 73 years) years and body mass index between 30 and 50 participated in a weight-loss program. Beliefs were measured at baseline by the Obesity Cognition Questionnaire and by an eating behavior self-efficacy scale. Correlational and regression analyses were performed to examine whether beliefs predicted weight change. Changes in body mass index, waist circumference, and blood pressure were significant (P <.001). Less weight reduction was associated with poor self-efficacy (r =-0.34, P <.01) and the beliefs that obesity had a physical origin (r =0.27, P =.04) and was not under behavioral control (r =-0.25, P =.04). Self-efficacy remained a significant predictor in regression analysis. The results suggest that the outcome of dietary interventions may be improved when adjusting beliefs, especially self-efficacy.