This study considered the role of dissociation and personality fragmentation as psychological factors that might distinguish borderline and personality disorder (BPD) patients, and that might explain why BPD patients have higher levels of other psychiatric symptomatology than those with other personality disorders. Two groups of personality-disordered patients (personality disorders including BPD; personality disorders other than BPD) completed measures of dissociation, personality fragmentation, and psychiatric disturbance. The BPD group had higher levels of a number of aspects of psychiatric symptomatology. Those differences were mediated by aspects of dissociation. Personality fragmentation differentiated the two groups but was not related to the higher levels of other aspects of psychiatric disturbance. Treating the syndrome of BPD may depend on addressing both dissociation and personality fragmentation. In contrast, reducing the level of comorbid psychiatric disturbance in BPD is likely to be more strongly dependent on reducing levels of different aspects of dissociation. Psychotherapeutic techniques and targets for achieving such change are discussed.