Objective. Researchers have turned to dimensional models of psychopathology as a means of explaining robust patterns of comorbidity. A hierarchical model consisting of internalizing and externalizing dimensions has been a useful approach to understanding comorbidity among some mental disorders, although a limited number of disorders have been examined within this framework. The objective of the present study is to determine how borderline personality disorder fits into this framework. Design and methods. Dimensional measures of nine psychiatric disorders were used in a confirmatory factors analysis to compare five roodels of comorbidity in 1,197 members (N = 541 women) of a population-based sample. Symptom composites were derived from the Michigan Composite International Diagnostic Interview and the International Personality Disorders Examination Questionnaire. Results. Five models were fit to dimensional indicators of nine disorders. A model in which borderline personality disorder served as a multidimensional indicator of the externalizing factor and the anxious-misery subfactor of internalizing disorders provided the best fit to the data in the whole sample and in men. For women, this model also fit well but an alternative model in which borderline personality disorder served only as an indicator of the anxious-misery subfactor of internalizing disorders fit equally well. Conclusions. The present study demonstrates the utility of the internalizing/externalizing framework for characterizing personality disorders as well as Axis I disorders. Future work should explore how other personality disorders fit into this framework.