Diagnostic categories are necessary, as are studies on relationships between the diagnostic categories of acute stress disorder and PTSD and whether these should be modified, retained, or combined (Brewin et al., 2003; reference 5). However, work on individual differences and case reports that illuminate the functional significance of specific symptomatic responses and their biological bases also advance our understanding of posttraumatic symptoms. Such work illustrates the heterogeneity of responses in PTSD and supports the notion of Foa and colleagues (6) that different PTSD symptoms, such as intense hyperarousal or numbing, may represent distinct pathological processes. Grouping PTSD subjects with different symptom patterns within the same diagnostic category can hinder our understanding of posttraumatic psychopathology. The heterogeneity of responses in PTSD may therefore shed light on the complexities of diagnosing and treating acute and enduring posttraumatic syndromes. We thank Dr. Clayton for raising important questions that gave us an opportunity to address these issues further.