While placed on different axes of the DSM classification system, borderline personality disorder (BPD) and posttraumatic stress disorder (PTSD) have important relationships with trauma, and overlap between these disorders has long been recognized. The current study is the first to examine comorbidity of PTSD and BPD in a large nationally representative sample using a reliable and valid method of assessing Axis I and II mental disorders. Data came from the National Epidemiologic Survey on Alcohol and Related Conditions (NESARC) Wave II (N = 34,653; response rate 70.2%). Multiple regression models were used to examine differences in psychopathology, traumatic events and health-related quality of life across individuals with PTSD alone (n = 1820), BPD alone (n = 1290) and those with comorbid PTSD-BPD (n = 643). The lifetime prevalence of PTSD and BPD were 6.6% and 5.9%, respectively. Of individuals with BPD, 30.2% were also diagnosed with PTSD, whereas 24.2% of individuals with PTSD were also diagnosed with BPD. Individuals with comorbid PTSD-BPD had a poorer quality of life, more comorbidity with other Axis I conditions, increased odds of a lifetime suicide attempt, and a higher prevalence of repeated childhood traumatic events than individuals with either condition alone. These results show that PTSD and BPD have a high degree of lifetime co-occurrence but are not entirely overlapping. Their concurrence is associated with poorer functioning compared to either diagnosis alone, emphasizing the clinical utility of diagnosing both conditions. Future research should explore the determinants of having either or both diagnoses with an aim toward improved identification, prevention, and intervention. Copyright © 2010 Elsevier Ltd. All rights reserved.