OBJECTIVE A subset of individuals with bulimia nervosa (BN) have borderline personality disorder (BPD) symptoms, including chronic negative affect and interpersonal problems. These symptoms predict poor BN treatment outcome in some studies. The broad version of Enhanced Cognitive Behavior Therapy (CBT-E) was developed to address co-occurring problems that interfere with treatment response. The current study investigated the relative effects, predictors, and moderators of CBT-E for BN with BPD and co-occurring mood/anxiety disorders. METHOD Fifty patients with BN and threshold or sub-threshold BPD and current or recent Axis I mood or anxiety disorders were randomly assigned to receive focused CBT-E (CBT-Ef) or broad CBT-E (CBT-Eb) specifically including an interpersonal module and additional attention to mood intolerance. RESULTS Forty-two percent of the sample reported remission from binge eating and purging at termination. Significant changes across symptom domains were observed at termination and at 6-month follow-up. Though CBT-Ef predicted good outcomes in multivariate models, the severity of affective/interpersonal problems moderated treatment effects: participants with higher severity showed better ED outcomes in CBT-Eb, whereas those with lower severity showed better outcomes in CBT-Ef. Severity of affective/interpersonal BPD symptoms at baseline predicted negative outcomes overall. Follow-up BPD affective/interpersonal problems were predicted by baseline affective/interpersonal problems and by termination EDE score. DISCUSSION This study supports the utility of CBT-E for patients with BN and complex comorbidity. CBT-Ef appears to be more efficacious for patients with relatively less severe BPD symptoms, whereas CBT-Eb appears to be more efficacious for patients with more severe BPD symptoms.