Little research to date has examined the ability of self-report measures to assess changes in symptom severity and diagnostic status as a function of treatment. This study investigated the validity of the posttraumatic stress disorder (PTSD) checklist (PCL) as a measure of symptomatic change following programmatic treatment. A sample of 97 Vietnam veterans with combat-related PTSD was assessed using the clinician-administered PTSD scale (CAPS) and the PCL prior to, and 9 months following, participation in a PTSD treatment program. Using the CAPS as the "gold standard" measure of PTSD symptomatology, the PCL demonstrated high diagnostic accuracy pre- and posttreatment. However, significant variations in accuracy were evident in the ability of the PCL to determine the presence and severity of individual symptoms at each time point. In addition, as symptoms improved from pre- to posttreatment, and approached the threshold criteria, the PCL demonstrated reductions in diagnostic accuracy. As a measure of overall symptomatic change, the PCL underrated improvement in comparison to the CAPS. The results supported the use of an overall cut-off score of 50 on the PCL for a diagnosis, and an item score of 3 for symptom criterion, in this population.