Obstructive sleep apnea (OSA) is a common sleep disturbance in individuals with posttraumatic stress disorder (PTSD), with an emerging literature showing that treating OSA with positive airway pressure (PAP) therapy has a moderate effect on decreasing PTSD severity. Unfortunately, PAP adherence among individuals with PTSD is low. Our study examined how baseline PTSD cluster subscores predict 6-month PAP adherence and how PAP adherence predicts change in PTSD cluster subscores over time. We examined PTSD and PAP adherence in 41 veterans with PTSD newly diagnosed with OSA over 6 months of PAP use (mean age = 50.27 years; 73.7% White; 13.6% female). The Posttraumatic Stress Disorder Checklist-Specific (PCL-S) was used to examine PTSD and subscales (re-experiencing, avoidance, and hyperarousal). We used longitudinal analyses to examine PTSD subscores on PAP adherence and PAP adherence predicting changes in PTSD subscores at 6-month follow-up. Among veterans with PTSD, higher levels of re-experiencing and hyperarousal, but not avoidance, predicted lower PAP use. Overall, the high-adherent group showed a 14.36-point decrease on the PCL-S, while the low-adherent group averaged just a 3.66-point decrease. More days of PAP use were associated with greater improvement in hyperarousal and avoidance subscores but not re-experiencing. Our findings reaffirm the importance of PAP use among patients with comorbid PTSD and sleep apnea, as well as the difficulty in achieving adherent PAP use in this population. Directly addressing heightened re-experiencing and hyperarousal in PTSD may increase PAP adherence among veterans with PTSD and requires future research. Colvonen PJ, Goldstein LA, Sarmiento KF. Examining the bidirectional relationship between posttraumatic stress disorder symptom clusters and PAP adherence. J Clin Sleep Med. 2023;19(5):857-863.