Objectives. Substance use disorder (SUD) and posttraumatic stress disorder (PTSD) frequently co-occur and having one condition worsens the course of the other. Individuals with both disorders exhibit worse functioning across a number of domains than individuals with either disorder alone. This is especially true in residential settings where both disorders are more severe than outpatient settings. Compared to Veterans with a single disorder, Veterans with SUD/PTSD also are more likely to have suicidal ideation and to have attempted suicide. Examining treatable conditions that are associated with improved SUD and PTSD outcomes, such as obstructive sleep apnea (OSA), can maximize treatment efficacy for Veterans at a critical time in recovery. OSA is highly comorbid with both PTSD and SUD with upwards of 67 to 83% of Veterans with SUD or PTSD also having OSA. Further, untreated OSA is associated with worse functional impairment across multiple domains, worse quality of life, worse PTSD, and higher substance use and relapse rates. Importantly, untreated OSA also contributes to higher suicide attempts and completion. Positive airway pressure (PAP) is the gold standard treatment for OSA with large effects on multiple domains of functioning, quality of life, PTSD symptoms, physical functioning, lower depression, and better emotional coping. Unfortunately, screening and treating Veterans for OSA is not a part of clinical practice for SUD or PTSD treatment; as such the average wait time for individuals to get PAP therapy is upward of two years. Despite the widespread dissemination of knowledge regarding the detrimental effects of untreated OSA and the incredible effectiveness of PAP treatment, OSA is rarely screened for or treated in patients with SUD or PTSD, with approximately 80% to 90% of Veterans with OSA remaining undiagnosed and untreated. Methodology. We aim to examine the effects of PAP treatment on Veterans with PTSD and SUD on a 28- day residential unit. We are proposing a randomized controlled study comparing two groups: an early intervention PAP treatment group receiving PAP treatment while on the residential unit, compared to a waitlist control group who will receive PAP treatment at 3-months post-discharge follow-up. Participants will be 194 male and female Veterans on the residential SUD and PTSD unit with SUD, PTSD, and OSA. Our primary aim is to determine the relative efficacy of PAP treatment on the SUD/PTSD unit, as compared to waitlist control, in reducing problematic substance use, PTSD symptoms, and suicidal ideation, while improving functioning among Veterans with comorbid SUD/PTSD at 3-months post-treatment follow-up. We will also compare PAP adherence rates on PTSD/SUD/functioning outcomes within the PAP treatment group (3-months). Finally, we plan on comparing adherence rates between the two treatment groups at the 6-months post-treatment follow- up assessment. Examining OSA treatment for SUD and PTSD is highly relevant to th...