Effectiveness of an integrated trauma-informed approach to treating women with co-occurring disorders and histories of trauma was evaluated. Baseline and 12-month assessments were completed by 136 intervention- and 177 comparison-group women. The intervention group received Seeking Safety, a trauma-specific group treatment focusing on safety and coping skills, in the context of integrated substance abuse and mental health services. The comparison group received similar services but not trauma-specific group treatment. Intervention women showed significantly better treatment retention over three months and greater improvement on posttraumatic stress symptoms and coping skills. On most outcomes, those who completed treatment improved more than those who discontinued. Improvements on symptoms of distress and drug problem severity were partially mediated by gains in coping skills. (Contains 2 tables and 3 figures.)