Sixty-one Vietnam veterans who had sought outpatient psychological services were evaluated for posttraumatic stress disorder (PTSD) during two independent diagnostic interviews. Data were analyzed from only those 48 subjects for whom the two diagnoses agreed upon the presence or absence of PTSD. Subjects were administered the Symptom Checklist-90-R, a modified version of the Impact of Event Scale, and two measures of combat stress: the Combat Scale Revised and the Vietnam Experience Scale. Some support was generated for the reliability and validity of the PTSD construct as outlined in DSM-III. In our sample the diagnosis of PTSD was associated with excessive arousal characterized by anxiety, anger, paranoid ideation, intrusive images, and avoidance of stimuli reminiscent of the traumatic stressor. The findings are discussed in relation to previous studies of combat-related PTSD and studies of traumatically stressed civilians.