The impact of extended longitudinal observation on the assessment of personality disorders. 
Multiple sources of information are necessary for a valid assessment of personality disorders (PDs).  This study investigates the impact of longitudinal observation.  The sample comprised 1217 patients from 15 different treatment units.  PDs were assessed at admission to treatment using the Structured Clinical Interview for DSM-IV Axis II Personality Disorders (SCID-II) and additional clinical information (best estimate diagnosis).  After approximately 18 weeks of treatment, the SCID-II protocols were re-examined at clinical conferences and the diagnostic status reassessed on the basis of longitudinal observations in multiple group situations (longitudinal, expert, all data principle).  Using this procedure, 78% of the patients' diagnostic criteria sets were changed, and 32% of patients' diagnostic statuses were changed.  Many (32%) patients who were evaluated initially as not having a PD received a PD diagnosis after re-examination.  The information provided by customary clinical assessment has important limitations, and longitudinal observation provides additional information that may change the diagnostic status in approximately one-third of PD cases.