BACKGROUND: A defining feature of personality disorder (PD) is an enduring pattern of inner experience and behavior that is stable over time. Follow-up and follow-along studies have shown considerable diagnostic instability of PDs, however, even over short intervals. What, then, about personality disorder is stable? The purpose of this study was to determine the stability of impairment in psychosocial functioning in patients with four different PDs, in contrast to patients with major depressive disorder (MDD) and no PD, prospectively over a 2-year period. METHOD: Six hundred treatment-seeking or treated patients were recruited primarily from clinical services in four metropolitan areas of the Northeastern USA. Patients were assigned to one of five diagnostic groups: schizotypal (STPD) (n=81), borderline (BPD) (n=155), avoidant (AVPD) (n=137), or obsessive-compulsive (OCPD) (n=142) personality disorders or MDD and no PD (n=85), based on the results of semi-structured interview assessments and self-report measures. Impairment in psychosocial functioning was measured using the Longitudinal Interval Follow-up Evaluation (LIFE) at baseline and at three follow-up assessments. RESULTS: Significant improvement in psychosocial functioning occurred in only three of seven domains of functioning and was largely the result of improvements in the MDD and no PD group. Patients with BPD or OCPD showed no improvement in functioning overall, but patients with BPD who experienced change in personality psychopathology showed some improvement in functioning. Impairment in social relationships appeared most stable in patients with PDs. CONCLUSION: Impairment in functioning, especially social functioning, may be an enduring component of personality disorder.