OBJECTIVE: This study reports the efficacy of a cognitive behavioral outpatient treatment on interpersonal outcome variables for patients diagnosed with borderline personality disorder. METHOD: In a 1-year clinical trial, 26 female patients with borderline personality disorder were randomly assigned to either dialectical behavior therapy or a treatment-as-usual comparison condition. All subjects met criteria of DSM-III-R and Diagnostic Interview for Borderline Patients for borderline personality disorder and were chronically suicidal. RESULTS: In both the intent-to-treat and treatment completion groups, dialectical behavior therapy subjects had significantly better scores on measures of anger, interviewer-rated global social adjustment, and the Global Assessment Scale and tended to rate themselves better on overall social adjustment than treatment-as-usual subjects. CONCLUSIONS: These results suggest that dialectical behavior therapy is a promising psychosocial intervention for improving interpersonal functioning among severely dysfunctional patients with borderline personality disorder.