PTSD is a common co-occurring disorder with substance use disorders (SUDs). When present, PTSD causes more negative outcomes in treatments designed to address substance use disorders. These negative outcomes include: 1) Early drop-out from treatment; 2) More frequent relapse to alcohol and other drugs; 3) Greater difficulty utilizing group therapies and self-help recovery groups; 4) Worsening of PTSD symptoms; and 5) Addiction treatment clinician distress. Nonetheless, the traditional practice in addiction treatment programs has been to address the problems of addiction, and delay treatment for PTSD and other psychiatric conditions. The evidence for negative outcomes for persons with co-occurring PTSD and SUDs challenges this practice, and suggests the need for new intervention strategies. A promising intervention for PTSD has emerged in another at-risk and vulnerable population of persons already receiving treatment for their "primary" disorder. This is a cognitive behavioral therapy for PTSD in persons with severe mental illness (SMI). Pilot studies with this intervention have demonstrated effectiveness, that it is well tolerated by patients, and that it fits within existing treatment services. Given the high prevalence of co-occurring PTSD and SUDs in addiction treatment settings (33-59%), the negative outcomes associated with this comorbidity, and the potential availability of a promising intervention, a stage one feasibility study appears warranted.