OBJECTIVE: Posttraumatic stress disorder (PTSD) and substance use disorders (SUDs) represent major public health concerns, particularly among veterans. They are associated with significant distress and impairment, and are highly comorbid. Little is known, however, about what role the temporal order of diagnostic onset may play in severity of presenting symptomatology and treatment outcomes. The aim of this study, therefore, was to examine treatment outcomes by order of onset. METHOD: Participants were 46 U.S. military veterans (91.3% male) enrolled in a larger randomized controlled trial examining the efficacy of an integrated, exposure-based treatment (Concurrent Treatment of PTSD and Substance Use Disorders Using Prolonged Exposure; COPE). Participants were grouped into 2 categories: (a) primary PTSD (i.e., PTSD developed before the onset of SUD) or (b) primary SUD (i.e., SUD developed before the onset of PTSD). RESULTS: No significant associations between order of onset and baseline symptomatology were observed. The findings revealed that participants with primary PTSD were significantly more likely than participants with primary SUD to report higher levels of PTSD symptoms at the end of treatment. However, there was no effect of order of onset on SUD outcomes. CONCLUSIONS: The findings suggest that individuals with earlier PTSD onset are a particularly high-risk group in terms of their trauma-related symptoms. Implications for treatment of comorbid PTSD/SUD are discussed. (PsycINFO Database Record (c) 2018 APA, all rights reserved).