||Within the Veterans Affairs Health Care System, posttraumatic stress disorder (PTSD) is routinely assessed in both mental health settings, including PTSD specialty clinics, and medical settings such as polytrauma clinics. To date, no empirical studies have been published that investigate the influence of contextual factors on measures such as the PTSD Checklist (PCL) when administered in different settings on separate, distinct occasions. Further, little is known about the impact on commonly used assessment measures when traumatic brain injury (TBI) and PTSD co-occur and what, if any, influence contextual factors have on veterans' endorsement of symptoms or their consistency in reporting the shared or overlapping symptoms. The present study conducted a systematic evaluation of the influence of measurement context on self-reported postconcussive and PTSD symptoms. Specifically, analyses focused on the consistency of reported symptoms across questionnaires and settings. Additionally, the influence of measurement occasion and context on PTSD symptom reporting within medical and mental health contexts was explored. Data from 713 Operation Enduring Freedom (OEF)/Operation Iraqi Freedom (OIF)/Operation New Dawn (OND) veterans was collected from retrospective medical chart reviews. Results indicated moderate to strong correlations between similar items measuring sleep, concentration, irritability, and loss of interest on the PCL, Neurobehavioral Symptom Inventory (NSI), and Patient Health Questionnaire (PHQ) and revealed no significant effect of context on item-level reporting. A fixed effects repeated measures ANOVA revealed no effect of measurement occasion and a two-period cross-over analysis revealed no evidence of differential carryover (e.g., there was no context X occasion interaction). However, the influence of context was significant and accounted for 12% of the variance in PCL scores. PCL mean scores were higher in the mental health setting (63.64) than the medical setting (58.37). This is the first study to the author's knowledge that investigated what, if any, differences are noted on PCL scores when administered in contextually different settings on distinct measurement occasions. Results suggest that self-reports of the severity of PTSD symptoms depend on whether the individual completes the PCL in a mental health (PTSD) or medical (polytrauma) context. Given the large numbers of recently returning OEF/OIF/OND veterans and the high prevalence rates of co-occurring PTSD and mild TBI, these results call for continued investigation of the reliability of commonly used assessment measures when administered in different contexts, and when diagnoses, especially those with many shared symptoms are co-occurring. Optimizing assessment protocols can help ensure accurate diagnosis and effective intervention, ultimately improving the overall quality of care for veterans.