Despite interest in the nature of the traumatic event required to meet Criterion A for posttraumatic stress disorder (PTSD) as defined in the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV; American Psychiatric Association, 1994), little attention has been paid to the diagnostic ramifications of linking PTSD symptoms to a single traumatic event in the context of multiple trauma exposures. In this study, 67 dually diagnosed clients with at least 2 potential Criterion A traumatic events completed the Posttraumatic Stress Diagnostic Scale twice, in counterbalanced order: once regarding their worst event and once regarding all events. When responding regarding their worst trauma, 53.7% met probable PTSD criteria. This rose to 67.2% when considering all traumas. Although preliminary, these results suggest that linking PTSD symptoms to a single traumatic event excludes a meaningful number of cases who are otherwise indistinguishable based on symptom profile.