Borderline personality disorder (BPD) is a severe psychiatric disorder that has a high clinical heterogeneity and frequent co-occurrence with other personality disorders (PDs). Although several studies have been performed to assess axis II comorbidity in BPD, more research is needed to clarify associated factors. The aim of this study was to determine the prevalence of co-occurrent axis II disorders in a large sample of patients with BPD and to investigate the influence of sex, age, and severity on this comorbidity. Data were collected from 484 patients with BPD through 2 semistructured interviews. We analyzed the frequency of axis II comorbidity and assessed differences regarding sex, age, and severity of BPD. About 74% of patients with BPD had at least 1 co-occurrent axis II disorder. The most common were paranoid, passive-aggressive, avoidant, and dependent PDs. Significant sex differences were found. Women presented more comorbidity with dependent PD, whereas men showed higher rates of comorbidity with antisocial PD. We also observed a significant positive correlation between age and the number of co-occurrent axis II disorders in women with BPD. Another finding was the positive correlation between BPD severity and the number of co-occurrent axis II disorders. These findings suggest that comorbidity with other axis II disorders and sex, age, and severity should be taken into account when developing treatment strategies and determining the prognosis of BPD. © 2011 Elsevier Inc.