Background Psychological and behavioral characteristics that predict success or failure with weight-loss treatments are poorly understood. Purpose The purpose of this study was to assess whether social cognitive theory-based factors discriminate between women who are successful and unsuccessful at weight loss. Method Obese women (BMI=30 to 45 kg/m2) who participated in a treatment of behavioral exercise support counseling and nutrition education were divided into quartiles based on percentage of body weight lost over 6 months. Factors based on social cognitive theory, both at baseline and change over 6 months, and exercise attendance were used to discriminate between the successful (highest quartile, Mchange in body weight=-9.3%; n=40) and unsuccessful (lowest quartile, Mchange in body weight=1.9%; n=37) groups. Results Stepwise discriminant analyses indicated that body satisfaction and tension (anxiety) scores at baseline, and changes over 6 months in self-regulatory efficacy and body satisfaction, made significant contributions to predicting group membership (64% and 69% of cases were correctly classified, respectively). Attendance percentage of exercise sessions was significantly greater for the successful weightloss group, and when added as a predictor, changes in selfregulatory efficacy and attendance made a significant contribution to predicting group membership (81% of cases were correctly classified). Conclusion Further research may enable psychological determinants to better guide weight loss theory and treatments. © 2010 International Society of Behavioral Medicine.