The influence of major depression on patients' and informants' reports of personality traits was examined using the Structured Interview for DSM-III Personality Disorder, both before and after successful antidepressant or placebo treatment (N = 58). According to patients' reports, Cluster A and C traits decreased significantly from pre- to posttreatment, but Cluster B traits were unchanged, excluding an increase in histrionic traits. According to informants' reports, Cluster A and B traits did not change from pre- to posttreatment, but Cluster C traits decreased significantly after treatment, not including passive-aggressive traits. Moreover, informants generally reported much higher levels of maladaptive personality traits than patients themselves. These results suggest that informants should be used in future research on personality disorders until better assessment techniques are developed.