Krueger [1999: Arch Gen Psychiatry 56:921-926] identified a three-factor structure of psychopathology that explained the covariation or grouping of common mental disorders found in the U.S. National Comorbidity Survey (NCS) [Kessler et al., 1994: Arch Gen Psychiatry 51:8-19]. These three fundamental groupings included an externalizing disorders factor and two internalizing disorders factors (anxious-misery and fear). We extended this research through the examination of additional data from a large subsample of the NCS (n=5,877) that contained diagnostic information on posttraumatic stress disorder (PTSD). Factor analytic findings revealed that PTSD showed no affinity with the fear factor defined by panic and phobic disorders, and instead loaded on the anxious-misery factor defined primarily by mood disorders. An identical pattern of results emerged for both lifetime PTSD and 12-month PTSD prevalence figures. Implications of these findings for the classification of PTSD and research on its etiology are briefly discussed.