STUDY OBJECTIVES to determine the short-term positive airway pressure (PAP) adherence rates and to identify non-mask-related risk factors associated with 30-day nonadherence to PAP in a population of veterans with obstructive sleep apnea (OSA) and posttraumatic stress disorder (PTSD). DESIGN a retrospective study. SETTINGS a Veterans Affairs hospital. PATIENTS one hundred forty-eight PTSD veterans newly diagnosed with OSA and a control group of OSA without PTSD matched for age, gender, BMI, and severity of OSA. INTERVENTIONS N/A. MEASUREMENTS AND RESULTS at 30-day follow-up, adherence to PAP was significantly lower in the PTSD group compared to the control group (41% versus 70%, respectively; P < 0.001). Veterans with adequate PAP adherence were more likely to experience sleepiness at baseline compared to nonadherent subjects (ESS 14.4 ± 5.3 versus 12.3 ± 5.9, respectively; P = 0.04). Nightmares were more frequently reported in those who were PAP nonadherent (P = 0.002). Mask discomfort, claustrophobia, and air hunger were the reported reasons for PAP nonadherence in the PTSD group. CONCLUSION PAP usage and adherence were lower in PTSD veterans with OSA than veterans without PTSD. Excessive sleepiness predicted PAP adherence while frequent nightmares were correlated with poor adherence to PAP therapy.