Abstract Introduction Obstructive sleep apnea (OSA) and PTSD are commonly comorbid in veterans. While PAP is the first line treatment for sleep apnea, adherence is low among veterans (Collen et al, 2012). Studies have shown early adherence predicts later adherence (Budhiraja et al, 2007). The current study investigates the impact of intervention on PAP adherence over time. Methods Thirty-seven veterans with comorbid OSA and PTSD from VA Palo Alto clinics were randomly assigned to either cognitive behavioral therapy (CBT-OSA) or an education arm and also received usual treatment at VA or outside clinics. Twenty-six completed the study. Participants received weekly, individual visits during the first four weeks of PAP treatment, three subsequent quarterly booster sessions, and a final visit at 12 months. PAP data was downloaded at each time-point; prior PAP use was coded. Results Significant mean differences in mask-on time between treatment arms after weekly treatments (Kinoshita et al., 2020) were not maintained at 12 months. Linear mixed modeling showed weekly improvement in mean mask-on time (minutes) over the treatment period (Education: estimate = 13.7, SE = 3.2, 95% CI 7.3, 20.1; CBT-OSA: estimate = 11.4, SE = 3.6, 95% CI 4.3, 18.5); no significant difference between arms (estimate = -2.3, SE = 4.7, 95% CI -11.5, 6.9). Newly diagnosed participants had significantly higher mean mask-on time than prior-PAP users at the end of weekly treatments (F[1, 35] = 17.86, p < 0.001) and at 12-months (F[1, 24] = 19.60, p < 0.001). Average mask-on time following weekly treatments was significantly correlated with average mask-on time at 12-months (r = 0.75; p < 0.001). Conclusion Consistent with prior research, findings suggest that newly diagnosed individuals benefit from weekly individual treatments, regardless of type, and maintain higher mask-on times at 12 months relative to prior PAP users. Early intervention when starting PAP treatment is important to support adherence, especially in patients with comorbid PTSD, a barrier to adherence in the veteran population. Support (if any) RR&D Merit Grant, Department of Veterans Affairs (Grant Number 1I01RX001799-01A2); Sierra Pacific MIRECC