Our ability to differentiate MPD from DSM-III-R Axis I disorders has become increasingly refined. Differentiation of MPD from the Axis II personality disorders is an area of more recent clinical investigation. MPD can be found comorbidity with many other psychiatric conditions. It is found in association with each of the DSM-III-R personality disorders. At the present time, however, we lack research data that define the prevalence of the comorbidity of MPD with the personality disorders. Objective study of this area is complicated by the paucity of instruments available to assess personality dimensions in the presence of a DD. In addition, the currently available personality inventories tend to overdiagnose BPD in patients with a high level of distress and acuity of symptoms. The diagnosis of a personality disorder in a patient with MPD is made on the basis of the assessment of the "whole" human being. It is based on the presence of a pervasive and relatively inflexible pattern of behaviors that reflects the individual predominant mode of being. The diagnosis of a personality disorder is not made on the basis of personality traits contained within any single alternate personality or groups of personalities. The personality disorders defined by DSM-III-R are a heterogeneous group of conditions whose individual etiologies reflect a complex interplay of constitutional, genetic, environmental, interpersonal, and psychodynamic factors. The interplay is variable and diverse between these determinants of the personality disorders and the traumatic forces that result in the development of a DD. For the Cluster A personality disorders (schizoid, schizotypal, paranoid), there is evidence supporting a relationship with specific psychotic illnesses. The combination of dissociative pathology with these personality disorders commonly results in a greater impairment of reality testing than in either condition alone. The Cluster B personality disorders (histrionic, narcissistic, borderline, antisocial) and Cluster C personality disorders (avoidant, compulsive, dependent, passive-aggressive) are believed to be primarily developmental disturbances. Comorbidity of these personality disorders with MPD involves consideration of the interaction of many developmental processes with the psychological impact of severe childhood trauma. Many MPD patients present with an apparent mixed personality profile consisting of an array of avoidant, compulsive, borderline, narcissistic, dependent, and passive-aggressive features. Although this article explores comorbidity of MPD with each of the personality disorders defined in DSM-III-R individually, it seems likely that a number of posttraumatic personality organizations can be defined that commonly coexist with MPD.(ABSTRACT TRUNCATED AT 400 WORDS)