This study investigated whether a subgroup of veterans with malignant posttraumatic stress syndrome, as described by Rosenheck (1985) and Lambert et al. (1996), could be identified via cluster analysis within two samples of Vietnam veterans with combat-related posttraumatic stress disorder (PTSD). In the initial subsample (n = 157), four clusters were identified, including a subgroup that scored consistently higher on measures of interpersonal violence and current physical problems. Similar results were found in the cross-validation subsample (n = 156). These results provide support for the theoretical concept of malignant PTSD and suggest that veterans with chronic PTSD are not homogenous. Whereas some manifest extreme levels of both functional impairment and PTSD symptomatology, others exhibit markedly less functional impairment despite manifesting clinically significant levels of PTSD. Clinicians can consider this heterogeneity in their treatment decisions. © 2002 Wiley Periodicals, Inc. J Clin Psychol 58: 1555–1571, 2002.