Cognitive-behavioral therapy (CBT) is an effective treatment of bulimia nervosa, but its mechanisms of action have not been established. In this study the authors analyzed the results of a randomized control trial comparing CBT with Interpersonal Psychotherapy (IPT) to identify possible mediators of change of CBT for BN and its time course of action. Reduction in dietary restraint as early as Week 4 mediated posttreatment improvement in both binge eating and vomiting. Measures of self-efficacy concerning eating behavior, negative affect, and body shape and weight at midtreatment were also significantly associated with posttreatment outcome at 20 weeks. No evidence was found that the therapeutic alliance mediated treatment outcome. CBT had a significantly more rapid treatment effect than IPT, with 62% of posttreatment improvement evident by Week 6.