Abstract:The unique nature of military-related trauma exposure, including a higher likelihood of multiple trauma exposures and sustained operational stress while deployed, may play a role in desensitizing the stress response system. Because the effectiveness of trauma-focused therapy is thought to rely on a heightened stress response, service members may not benefit from exposure therapy as much as civilian populations. Research on stress response system functioning in the military PTSD population is limited. The goals of this study were to examine cortisol awakening response (CAR) as well as cortisol reactivity as predictors and correlates of PTSD treatment outcomes among service members who participated in an experimental PTSD treatment program. Additionally, I explored if cortisol levels at baseline predict treatment attrition. Linear mixed models were used to evaluate baseline CAR and cortisol reactivity as predictors of PTSD treatment outcomes. Cross-lagged panel models were used to assess the relationship between cortisol levels and PTSD symptom severity over time. Finally, logistic regression and survival analysis were used to examine if cortisol level at baseline were associated with treatment attrition by end-of-study and time-to-attrition during treatment. No significant relationships were found between CAR or cortisol reactivity and PTSD outcomes. CAR was a significant predictor of attrition (β = 0.40, σ = 0.15, p < .01) and time-to-attrition (β = -0.31, σ = 0.12, p < .01). This research adds to the scarce research exploring HPA axis dysregulation in the military population. In particular, it underscores the association between HPA axis functioning and treatment completion.