Background: Evidence-based treatments for complex posttraumatic stress disorder (PTSD) related to childhood abuse are scarce. This is the first randomized controlled trial to test the efficacy of psycho-educational and cognitive behavioural stabilizing group treatment in terms of both PTSD and complex PTSD symptom severity. Methods: Seventy-one patients with complex PTSD and severe comorbidity (e.g. 74% axis II comorbidity) were randomly assigned to either a 20-week group treatment in addition to treatment as usual or to treatment as usual only. Primary outcome measures were the Davidson Trauma Scale (DTS) for PTSD and the Structured Interview for Disorders of Extreme Stress (SIDES) for complex PTSD symptoms. Statistical analysis was conducted in the intention-to-treat (ITT) and in the completer sample. Subjects were considered responders when scoring at 20 weeks at least 1 standard deviation below pretest findings. Results: The 16% attrition was relatively low. After 20 weeks, the experimental condition (large effect sizes) and control condition (medium effect sizes) both showed significant decreases on the DTS and SIDES, but differences between the conditions were not significant. The secondary responder analysis (ITT) revealed significantly more responders on the DTS (45 vs. 21%), but not on the SIDES (61 vs. 42%). Conclusions: Adding psycho-educational and cognitive behavioural stabilizing group treatment for complex PTSD related to child abuse to treatment as usual showed an equivocal outcome. Patients in both conditions improved substantially during stabilizing treatment, and while significant superiority on change scores was absent, responder analysis suggested clinical meaningfulness of adding group treatment. Copyright © 2012 S. Karger AG.