Little is known about the relative severity or typical sequence of Diagnostic and Statistical Manual (DSM-IV) symptoms of posttraumatic stress disorder (PTSD). Using data from the National Comorbidity Study-Replication (NCS-R) [Kessler, R. C., Berglund, P., Chiu, W. T., Demler, O., Heeringa, S., & Hiripi, E., et al. (2004). The US National Comorbidity Survey-Replication (NCS-R): design and field procedures. International Journal of Methods in Psychiatric Research, 13(2), 69-92], the current study used a logistic item response model to assess the degree to which DSM-IV symptoms combine to define a primary construct underlying PTSD, to identify which symptoms are associated with greater severity of PTSD, and to determine whether the symptoms and symptom patterns are influenced by gender. Results suggested that PTSD symptoms can be combined to assess a single dimension of PTSD severity, providing support for a continuum of symptom severity. However, several DSM-IV symptoms provided overlapping information, potentially reducing the effectiveness of these symptoms in describing a broad range of PTSD. More precise assessment of PTSD severity may help improve the descriptive value of PTSD measures relationship to continuous measures of treatment outcomes, and ultimately inform more effective treatments.