To the Editor: We are pleased that Dr. Clayton raised the interesting issue of how best to interpret our finding that acute stress disorder and PTSD can be commonly diagnosed simultaneously, provided that the time criterion requiring PTSD symptoms to have been present for 1 month is ignored. This observation was illustrated by the patients described by Dr. Lanius et al. Our own conclusions are somewhat different from those of Dr. Clayton. In our opinion, our data make it hard to sustain the position that acute stress disorder and PTSD are two distinct disorders. We prefer to think that there is only one disorder but that specific processes may be present or absent and have a corresponding impact on pathophysiology. There are now several studies that implicate peritraumatic dissociation in explaining the variability in the psychophysiological reactions of patients with PTSD and suggest that such dissociation may reflect a freezing response as opposed to a fight-or-flight response (1). There are a number of reasons why one person responds to exposure therapy and another does not. For example, negative emotions, such as shame (2), and negative beliefs about the trauma or about subsequent symptoms (such as dissociation) have been shown to impede recovery. Our guess is that the payback will be greater from understanding these processes than from further refining our diagnoses.