<p>To evaluate the diagnostic utility of the Structured Inventory of Malingered Symptomatology (SIMS) for differentiating instructed feigned PTSD, from a Ministry of Defense (MoD)-recognized PTSD group (Israel), in comparison with healthy controls, and to examine whether PTSD symptom severity and neuroticism contribute to SIMS misclassification. Participants (<i>N</i> = 112) were Israeli men and included a MoD-recognized PTSD group (<i>n</i> = 38), healthy controls (<i>n</i> = 34) and instructed feigning group (<i>n</i> = 40). Measures included the SIMS, Test of Memory Malingering (TOMM), PTSD Checklist for DSM-5 (PCL-5), and Big Five Inventory (BFI). Using the Israeli-recommended SIMS cutoff (&gt; 20), sensitivity for detecting instructed feigning was 85%, whereas specificity within the MoD-recognized PTSD group was 61%, indicating a substantial false-positive rate. All healthy controls were classified below cutoff (100% specificity). On the TOMM, all healthy controls and all MoD-recognized PTSD participants demonstrated valid performance (100% specificity), whereas 70% of the instructed feigning group were also classified as providing valid performance. Within the MoD-recognized PTSD group, PTSD symptom severity was positively correlated with both SIMS total score and the Affective subscale (AF) (for both associations: r = .62, <i>p </i>&lt; .01). Neuroticism was also positively correlated with PTSD symptom severity (r = .54, <i>p </i>&lt; .01) and SIMS total score (r = .50, <i>p</i> &lt; .01). In this Israeli sample, the SIMS showed high sensitivity for identifying invalid responding within the instructed-feigning paradigm but limited specificity in a MoD-recognized PTSD group, yielding a substantial false-positive rate. SIMS results should be interpreted cautiously in longstanding PTSD, particularly when symptom severity is high, and ideally considered within a multi-method validity framework.</p>