OBJECTIVE: The aim of this study was to prospectively examine the relationship between immediate and short-term responses to a trauma and the subsequent development of posttraumatic stress disorder (PTSD). METHOD: All patients consecutively admitted to a general hospital were screened for the presence of physical injury due to a traumatic event. Fifty-one eligible subjects were assessed 1 week and 6 months after the trauma. The initial assessment included measures of event severity, peritraumatic dissociation, and symptoms of intrusion, avoidance, depression, and anxiety. The follow-up assessments added the PTSD module of the Structured Clinical Interview for DSM-III-R--Non-Patient Version and the civilian trauma version of the Mississippi Scale for Combat-Related Posttraumatic Stress Disorder. RESULTS: Thirteen subjects (25.5%) met PTSD diagnostic criteria at follow-up. Subjects who developed PTSD had higher levels of peritraumatic dissociation and more severe depression, anxiety, and intrusive symptoms at the 1-week assessment. Peritraumatic dissociation predicted a diagnosis of PTSD after 6 months over and above the contribution of other variables and explained 29.4% of the variance of PTSD symptom intensity. Initial scores on the Impact of Event Scale predicted PTSD status with 92.3% sensitivity and 34.2% specificity. Symptoms of avoidance that were initially very mild intensified in the subjects who developed PTSD. CONCLUSIONS: Peritraumatic dissociation is strongly associated with the later development of PTSD. Early dissociation and PTSD symptoms can help the clinician identify subjects at higher risk for developing PTSD.