This study explored the physiological responses of posttraumatic stress disorder (PTSD) patients to reminders of a stressful event that had preceded the onset of their illness and was not related to its cause: the SCUD missile alarms of the Gulf War. A mental-imagery technique used in previous studies of PTSD was used. Three 30-second audiotapes were presented to each subject, including (1) the Gulf War's missile alarm, (2) a radio announcement of a terrorist attack, and (3) a standardized relaxing scene. Subjects were instructed to imagine each event as vividly as possible while heart rate (HR), skin conductance (SC), and left lateral frontalis electromyogram (EMG) responses were measured. The responses of 12 outpatients with PTSD were compared with those of panic disorder patients (n = 11), survivors of traumatic events who had not developed PTSD (n = 9), and mentally healthy subjects with no lifetime history of major trauma (n = 19). Multivariate analysis of variance (MANOVA) for the three physiological measures showed a significant group difference during imagery of the Gulf War alarm, with PTSD subjects showing higher SC and EMG responses than the others. The differences remained significant when age, level of distress during the war, and concurrent anxiety were controlled for. There were no group differences in responses to the other stimuli. We conclude that, PTSD patients may either acquire and maintain prolonged conditioned responses to various stressors during their life span or become sensitized to reminders of past traumata following the onset of their illness. Heightened conditionability may be expressed before the trauma in subjects who are liable to develop PTSD.