Background. Social dysfunction in personality disorder is commonly ascribed to abnormal temperamental traits but may also reflect deficits in social processing. In this study, we examined whether borderline and avoidant personality disorders (BPD, APD) may be differentiated by deficits in different social domains and whether disorganization of social domain functioning uniquely characterizes BPD. Method. Patients were recruited from psychiatric clinics in Pittsburgh, USA, to provide a sample with BPD, APD and a no-personality disorder (no-PD) comparison group. Standardized assessments of Axis I and Axis II disorders and social domain dysfunction were conducted, including a new scale of 'domain disorganization' (DD). Results. Pervasive social dysfunction was associated with a 16-fold increase in the odds of an Axis II disorder. Both APD and BPD were associated with elevated social dysfunction. Romantic relationship dysfunction was associated specifically with BPD symptoms and diagnosis. DD was associated specifically with a categorical BPD diagnosis and with a dimensional BPD symptom count. Conclusions. A focus on the inherently interpersonal properties of personality disorders suggests specific mechanisms (within and across interpersonal domains) that may help to account for the origins and maintenance of some disorders. In particular, BPD reflects disturbances in romantic relationships, consistent with a role for attachment processes, and in the organization of functioning across social domains. © 2007 Cambridge University Press.