The authors investigated the relationship between personality disorders and treatment outcome in the National Institute of Mental Health Treatment of Depression Collaborative Research Program, which involved 239 outpatients with major depressive disorder randomly assigned to one of four 16-week treatment conditions. Patients with personality disorders (74% of the sample) had a significantly worse outcome in social functioning than patients without personality disorders and were significantly more likely to have residual symptoms of depression. There were no significant differences in work functioning or in mean depression scores at treatment termination. Outcome was similar for patients in the different clusters of personality disorders.