The authors evaluated the validity of the Post-Traumatic Stress Disorder (PTSD) subscale of the Minnesota Multiphasic Personality Inventory (MMPI), the Impact of Event Scale (IES) and the Symptom Check List 90 (SCL-90) as continuous and dichotomous measures of PTSD in a mixed military and civilian group of 70 subjects in the United Kingdom. The MMPI-PTSD and the IES are designed specifically as measures of PTSD and the Global Symptom Index of the SCL-90 is a general measure of neurosis. All measures produced significant positive correlations with scores from the Clinician Administered Post-Traumatic Stress Disorder Scale (CAPS-1) and with each other. The IES was the most useful dichotomous measure. The optimum cut-off score for the IES producing the highest Positive Predictive Value and the lowest Apparent Total Misclassification Error Rate has been determined.