A post-traumatic stress disorder (PTSD) scale for the Millon Clinical Multiaxial Inventory (MCMI) was developed using an outpatient sample consisting of 45 clients meeting criteria for the PTSD syndrome in accordance with the Diagnostic and Statistical Manual (DSM-III), and an equal number of matched controls. The scale was derived empirically by looking at each MCMI item and determining if there was any difference in the way the individuals from the two groups responded to the particular item. The method led to 17 items that diagnosed PTSD subjects in the derivation sample with accuracy. The scale was then tested against a second cross-validating sample. The latter sample consisted of 43 patients admitted into a PTSD inpatient unit for treatment; the control group used a matched sample of psychiatric inpatients without PTSD. Although the results were significant, the mean and standard deviations of the two cross-validating groups were such that it may be difficult to use the scale clinically. The possibility that a better scale could be developed choosing items on the basis of face validity was explored. (Author/SLD)