Posttraumatic stress disorder (PTSD) and alcohol use disorder commonly co-occur. Little is known about how symptoms of one affect subsequent week symptoms of the other during the course of integrated treatment for both disorders. The sample included 107 veterans who were randomized to receive either Concurrent Treatment of PTSD and Substance Use Disorder Using Prolonged Exposure (COPE; an exposure-based trauma focused treatment) or Seeking Safety (SS; a present-focused coping skills-based treatment) and completed measures of PTSD and alcohol use at every other session. Multilevel models estimated the prospective associations between PTSD and alcohol use during treatment. Results indicated that greater PTSD symptom severity was associated with greater future alcohol use (b = 0.20, p = .024), and greater alcohol use was associated with greater future PTSD symptom severity (b = 0.13, p = .003). The effect size for PTSD symptoms to future alcohol use was larger than the reciprocal relationship. When using lagged PTSD severity to predict future drinking, results revealed that clinically significant differences in PTSD severity levels were associated with comparably large differences in drinking. Treatment condition did not moderate the effect of PTSD symptom severity on alcohol use (or the reciprocal relationship). Findings lend support to the mutual maintenance model of addiction. Integrated treatments that treat both PTSD and alcohol use may be preferential to sequential model of care where individuals are expected to achieve abstinence or reduced use prior to receiving trauma-focused treatment. (PsycInfo Database Record (c) 2020 APA, all rights reserved).