Despite growing support for the use of imagery rescripting to treat posttraumatic nightmares (PTNMs), its underlying mechanisms have not been examined. This secondary data analysis piloted the proposal that modification of posttraumatic cognitions is a mechanism of change when using a manualized PTNM imagery rescripting intervention. Significant linear reductions in posttraumatic cognitions were observed from baseline through 6-month follow-up evaluations. Change in total negative cognitions was significantly correlated with change in posttraumatic stress disorder symptoms. Initial amount of change in subscale scores also predicted the amount of distal change observed at the 6-month follow-up. These findings provide preliminary evidence that trauma-related cognitions may improve over time as a result of imagery rescripting. © 2011 Wiley Periodicals, Inc. J Clin Psychol 67:1–9, 2011.